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Each autumn, nearly the entire breeding population of the Swainson’s Hawk migrates from the temperate zone of North America to “wintering” areas in South America. From prairie Canada, this migration is more than 10,000 km each way, a distance second among raptors only to that of the Arctic Peregrine Falcon (Falco peregrinus tundrius).

A highly gregarious species, the Swainson’s Hawk forages and migrates in flocks sometimes numbering in the thousands. Its movement through Central America has been described as among “the most impressive avian gatherings in North America, since the demise of the Passenger Pigeon” (Brown and Amadon 1968). Nearly 350,000 Swainson’s Hawks have been counted passing over a single point in Panama City in October and November, and up to 845,000 have been counted in a single autumn in Veracruz, Mexico.

Although wide-ranging and common, the discovery and naming of this species did not come about until the 1820s. Dr. John Richardson, English surgeon and naturalist with the Franklin Arctic expeditions, collected the first Swainson’s Hawk in 1827 at Fort Carlton near Saskatoon, Saskatchewan. The bird was illustrated by William Swainson and identified as Buteo vulgaris (now Buteo buteo, the Common Buzzard of Europe) in Richardson and Swainson’s classic work Fauna Boreali-Americana . In 1838, Charles Lucien Bonaparte, realizing this was a different species, applied the name Buteo swainsoni, commemorating Swainson’s earlier work, but basing his description on a plate drawn by John James Audubon for a bird collected at Fort Vancouver, Washington.

The breeding-season diet of the Swainson’s Hawk is similar to that of other temperate-zone buteos; young are fed rodents, rabbits, and reptiles. When not breeding, however, this hawk is atypical because it is almost exclusively insectivorous, eating grasshoppers (Acrididae) in particular. Only by concentrating on abundant insects can large concentrations of Swainson’s Hawks be sustained.

In many parts of its range, this hawk has adjusted to agricultural landscapes. Nonetheless, its numbers have declined significantly in parts of the western United States; and in the western Canadian prairie, reproduction of the Swainson’s Hawk has dropped since the mid-1980s, following a decline in its main prey species, Richardson’s ground squirrel (Spermophilus richardsonii). In 1995, satellite telemetry was used for the first time to investigate Swainson Hawk migration; these studies revealed that thousands of these hawks were dying in Argentina as a result of the use of pesticides

Description

Like other members of genus Buteo, relatively stout-bodied and broad-winged. Total length of males 48–51 cm, mass 693–936 g; females, 51–56 cm long, mass 937–1,367 g (J. K. Schmutz in Palmer 1988). Sexes similar in appearance, although distinguishable by size, mainly using body mass, wing chord and foot pad length as predictors Kochert and McKinley 2008) or, alternatively, forearm length, wing chord and tail length Sarasola and Negro 2004); polymorphic (dark to light or rufous; see Appearance, below).

Identification

Compared to other North American buteos, overall appearance slimmer, with thinner body and narrower wings. In flight, holds wings in dihedral during soar; dark flight-feathers contrast with paler wing-linings (pale morphs); long wings tapered, pointed, and in perched birds reach or barely exceed tip of tail. Tail grayish to grayish brown with numerous narrow, dark bands; subterminal band widest, and tip of tail white to buffy in fresh plumage. Tail slightly lighter toward base; combined with white barring on uppertail-coverts, can make base of tail appear whitish, especially in flight. Pale morph birds show dark breast-band, or “bib,” between lighter belly and chin; light- to intermediate morphs show pale forehead when viewed at extremely close range. Dorsal surface relatively evenly colored compared with most other buteos.

Darkest adult Swainson’s Hawks lack sharp contrast between wing-linings and flight-feathers, and entire breast and belly can be nearly uniform dark brown to grayish or blackish brown; tail darker, but pattern same as on lighter-colored birds; undertail-coverts buffy white, barred with dark brown. Juvenile Swainson’s Hawks have similar underwing pattern as adult of same morph; underparts streaked with large, globular spotting on breast, usually heaviest at sides; streaking usually extends to belly; light supercilium and cheeks; dark malar mark; back mottled or streaked. Later subadult plumages similar to juvenile but frequently show much paler, often whitish, head (Wheeler and Clark 1995).

At all ages, most likely to be confused with Broad-winged (Buteo platypterus), White-tailed (B. albicaudatus), and Short-tailed (B. brachyurus) hawks. Among buteos regularly found in U.S. and Canada, on Swainson’s, Broad-winged, and White-tailed hawks, outer 3 primaries are emarginate (inner web narrows abruptly or is incised toward tip); Short-tailed Hawk has 3 emarginate and fourth primary sinuate; on all others, outer 4 primaries are emarginate (Friedmann 1950). Among species with 3 emarginate primaries, Swainson’s and White-tailed have longer wings (>360 mm). White-tailed has very short tail, however: less than half the wing length (Friedmann 1950). Legs of Swainson’s Hawks half-feathered, in contrast to fully feathered legs of Ferruginous (Buteo regalis) and Rough-legged (B. lagopus) hawks.

Short-tailed Hawk lacks dark breast-band, and its primaries have whitish bases. White-tailed Hawk lacks dark breast-band and has whitish tail with bold black subterminal band and white uppertail-coverts. Adult Broad-winged Hawk has broad black and white bands on tail; at all ages Broad-winged has entirely pale underwing (light birds) or dark wing-linings contrasting with light-colored primaries and secondaries (the reverse of Swainson’s), and dark undertail-coverts (dark birds). Swainson’s Hawk also distinguished from dark-morph adult White-tailed Hawk by tail pattern and from dark-morph Short-tailed Hawk by dark undertail-coverts and contrast between dark wing-linings and lighter-colored primaries and secondaries. All other dark-morph North American buteos have dark undertail-coverts. Juvenile light- to intermediate-morph Short-tailed Hawk has only fine, dark streaking on sides of upper breast, secondaries are darker than primaries, and tail has fine evenly spaced bars. Juvenile light-morph Red-tailed Hawk (Buteo jamaicensis) also has dark patagial marks. Light-colored undertail-coverts and evenly colored primaries and secondaries of dark-morph Swainson’s Hawk distinguish it from all other dark-morph juvenile North American buteos except White-tailed Hawk. Dark hatch-year and first year White-tailed Hawk has heavily streaked dark and light on abdomen, white breast-patch, and white uppertail-coverts, creating distinct U above base of tail. Perched light-morph juvenile Swainson’s Hawks can be very similar to Prairie Falcon (Falco mexicanus), but latter has dark patch behind eye and wingtips do not reach end of tail. See Clark and Wheeler 1987, Palmer 1988, and Wheeler and Clark 1995 for additional information on identification

Breeding Range

Historically and in existing native habitat, forages in open stands of grass-dominated vegetation, sparse shrublands, and small, open woodlands. In many parts of range today, has adapted well to foraging in agricultural areas (e.g., wheat and alfalfa), but cannot forage in most perennial crops or in annual crops that grow much higher than native grasses, making prey more difficult to find (Bechard 1982, Estep 1989, Woodbridge 1991). In Central Valley, CA, forages in row, grain, and hay crop agriculture, particularly during and after harvest, when prey are both numerous and conspicuous; also attracted to flood irrigation, primarily in alfalfa fields, when prey take refuge on field margins, and to field burning, which forces prey to evacuate (J. A. Estep pers. comm.). In ne. California, 42.5% of habitat within foraging radius of 12 radio-marked individuals was in active agriculture (Woodbridge 1991). In Utah, nest sites contained significantly more pasture and occurred only on level terrain in the valley floor (Bosakowski et al. 1996). In North Dakota, 75.4% of area within 1 km of nests (n = 27) was either pasture or hayland, and only 17.7% was cultivated crops; only 2 pairs nested where >60% was cultivated crops (Gilmer and Stewart 1984). In e. Washington, home ranges consist of 25.2% grassland, 50.4% wheat, 17.2% shrub vegetation, and 7.2% other (Bechard et al. 1990). In contrast, species appears to increase in density in Alberta as cultivation increases to 30% of home range, but there is no further change in density with additional increases in cultivation (Schmutz 1989).

Typically nests in scattered trees within these grassland, shrubland, or agricultural landscapes (e.g., along stream courses or in open woodlands). In plains of w. Canada and northern states of U.S., in nineteenth century, major fires burned grasslands every few years, keeping trees to a minimum. An occasional pair nested on the ground, though such nests were subject to trampling by American bison herds (Bos bison), so surviving small willows (Salix spp.) and low aspen (Populus spp.), chiefly along and around water bodies, were used whenever available. In N. Dakota, large majority of nest trees found in planted shelterbelts (43%), wetland borders (22%), and abandoned farmsteads (11% ; Gilmer and Stewart 1984). Today, in California’s Central Valley, nests are typically at edge of narrow bands of riparian vegetation, in isolated oak woodland, and in lone trees, roadside trees, or farmyard trees, as well as in adjacent urban residential areas (Estep 1989, England et al. 1995). When coexisting with Red-tailed Hawk, Swainson’s uses smaller trees in smaller clumps than does Red-tailed (p ≤ 0.001; Murphy 1993). A few pairs also nest in northern Mexico ranging from 12 pairs in the Mapimi Desert in northern Durango (Rodriguez-Estrella 2000) to 2 pairs in Neuvo Leon (Contraras-Balderas and Monteil-de-la-Garza 1999). See also Breeding: nest site, and Food habits: feeding, below.

Spring And Fall Migration

Birds rest and feed in grasslands and harvested fields, especially where grasshoppers are numerous, often perching on fence posts, telephone poles, and power poles. Large flocks may roost at night in trees (CSH, Smith 1980).

Winter Range

Historically in Argentina during austral summer, inhabited native grasslands similar to those of n. Great Plains (Fig. 3). Now, as in much of breeding range, species has adapted to agriculture; in e. La Pampa province of Argentina, typically found where alfalfa is grazed by cattle and where sunflowers and corn are abundant (Woodbridge et al. 1995b). However, hawks only used plowed fields and permanent pastures such as fallow, natural, and alfalfa fields to forage (Canavelli et al. 2003). In e. Buenos Aires Province, found in mosaic of salt marsh, freshwater marsh, and pampas (Rudolph and Fisher 1993). Regional scale distribution in wintering grounds is determined by land use types, topography and climate (Sarasola et al. 2008).

Probability of occurrence in each of 30 x 30 km squares in which the Argentine pampas was divided was positively related with the percentage of land devoted to cereal crops (wheat, rye and barley) and perennial pastures (mainly alfalfa) but negatively affected by the percentage of land devoted to oleaginous crops (mainly soy bean) and annual pastures (oat, sorghum). In addition, probability of occurrence of Swainson’s Hawks is higher at intermediate altitudes (200 m a.s.l.) avoiding lowlands and highlands. Drier conditions during austral springs (time in which hawks arrive at the wintering quarters) also increase local probability of occurrence of hawks. Abundance of Swainson’s Hawks at this regional scale is also influenced by land use types and climate.

Similarly, hawk abundance was positively and negatively affected by perennial and annual pastures respectively. Abundance higher when natural grasslands are present and also at intermediate values of percentage of land implanted with groves of exotic tree species. Swainson’s Hawk local abundance is also higher when both austral spring and austral summer of the previous year are especially drier. This could be related to insect abundance in the field because in temperate areas such as Argentina, most grasshopper species emerging in any given year laid their eggs during the previous summer.

At night, perches in eucalyptus (Eucalyptus spp.) groves or “montes” planted as shelterbelts or windbreaks around farm sites (Woodbridge et al. 1995b, MJB). A survey of roost sites throughout the Argentine pampas showed hawks using only groves of exotic tree species for roosting (Sarasola and Negro 2006), including eucalyptus but also elm and pine. These groves of exotic tree species are novel elements on the Argentine pampas landscapes, which lacked much of any arbores cent growth before European colonization during the 20th century. Thus, the introduction of exotic trees may have resulted either in the expansion of the suitable habitat for Swainson’s Hawks, permitting a recent colonization of the Argentine Pampas, or a change in the communal roosting behavior of hawks when these structures become available.

Segregation On Wintering Grounds Among Age-Sex Classes And/Or According To Breeding Origins

Needs study. In e. La Pampa Province, Argentina, foraging flocks are in pampas and agricultural areas and are dominated by adult birds (Woodbridge et al. 1995b, A. Lanussé pers. comm.). In contrast, flocks in e. Buenos Aires Province are in wet pampas dominated by juveniles (Jaramillo 1993, pers. comm.). Stable isotope analysis reveals wintering aggregations of hawks at roosting places consists of a mixture of individuals originating from different breeding sites in North America (Sarasola et al. 2008b). Values of stable-hydrogen isotope were measured for 40 hatching year old Swainson’s hawks captured at five roosting sites across the Argentine pampas. Variability in the values of isotope signatures in feather of hawks captured at the same roosting place were 11 to 19 times greater than that expected in birds with the same breeding origin.

Nature Of Migration In The Species

Nearly the entire population migrates annually between breeding areas in North America and wintering grounds in pampas of South America, a round-trip that can exceed 20,000 km. Individuals are absent from breeding grounds for 5–6 mo in central California, 205–217 d in ne. California (Woodbridge 1991), and 7–8 mo in Saskatchewan (CSH). Generally migrate in flocks that can be as large as 5,000–10,000 individuals, always during daylight, typically soaring in thermals, and rarely over water (Fig. 2). Although overall migration period at any given location in spring or fall may last 2 mo, most individuals pass through Central America during brief, concentrated period (roughly 2 wk) during both seasons (Smith 1980, Ruelas Inzunza et al. 1993). In Mexico and Central America, associates with flocks of Broad-winged Hawks, Turkey Vultures (Cathartes aura), and Mississippi Kites (Ictinia mississippiensis), creating a river of hawks called “one of the most spectacular and easily observed movements of birds in the New World, and possibly anywhere” (Smith 1980: 52). No known sexual differences in migration pattern or timing.

Timing And Routes Of Migration

Fall

Flocks of several hundred immatures and postbreeding adults gather by late Aug and early Sep, fattening on grasshoppers (Orthoptera; McGrath 1988, Houston 1990). With aid of northerly winds, they then travel south in flocks. In 1995, 2 adult hawks (1 female and 1 male) with satellite transmitters were tracked from their territories near Hanna, Alberta, to Argentina. On 14 Sep 1995 these birds were on their territories; reached sw. Saskatchewan by 23 Sep. The male (position transmitted weekly) passed through Nebraska on 1 Oct; Tamaulipas, Mexico, on 7 Oct; Honduras on 14 Oct; and reached Marcos Juárez, Argentina, on 7 Nov. The female (position reported twice weekly) reached Kansas on 5 Oct; Tamaulipas, Mexico, on 9 Oct; Oaxaca, Mexico, on 12 Oct; Nicaragua on 19 Oct; Panama on 22 Oct; passed through Colombia on 25–28 Oct; south of Santa Cruz, Bolivia, on 4 Nov; and reached Armstrong, Argentina, on 7 Nov. Male took up to 54 d, averaging 194 km/d, while female took up to 53 d and averaged 198 km/d. During two 3-d periods in early Nov, female averaged 565 km/d and 473 km/d, respectively. In the month after arrival, female continued another 650 km south within Argentina (Schmutz et al. 1996).

In 1996 and 1997, 34 Swainson’s hawks were captured throughout the breeding range in North America and instrumented with satellite radios Fuller et al. 1998, (Bechard et al. 2006). Only 27 of these completed the southward migration with a mean cumulative distance of 13,504 km and a mean direct distance of 10,139 km, averaging 188 km/day. They converged in eastern Mexico on the Gulf of Mexico coast, then followed a narrow, well-defined path through Central America, across the Andes Mountains in Columbia, and east of the Andes to central Argentina where they all spent the austral summer (Fuller et al. 1998).

Band recovery data and ground observations give comparable migration dates. Most individuals have left British Columbia by late Aug or early Sep (recorded to 30 Oct; Campbell et al. 1990). Transients observed mid-Aug–early Oct in s. California (extremes late Jul–late Oct; Garrett and Dunn 1981); late Aug–early Oct, peaking in late Sep in Minnesota (recorded to 7 Nov; Janssen 1987); late Aug through third week of Oct, peaking in late Sep or early Oct, in Missouri (Robbins and Easterla 1992); Oct–Nov in Mexico (Howell and Webb 1995); Oct–early Nov in Panama (occasionally late Sep; Smith 1985a, Ridgely and Gwynne 1989); and Sep–early Nov in Colombia (occasionally early Aug; Hilty and Brown 1986). Earliest band recovery in Guatemala was 7 Oct, in Colombia 22 Oct, and in Argentina 10 Nov (Houston and Schmutz 1995b).

Migration proceeds on broad front through s. Canada and U.S., but as birds converge in Middle America, huge flocks form as many birds pass through much narrower areas. On 6 Oct 1953, 16 km west of San Antonio, TX, 25,000 birds passed over 0.4-km observation strip; flight was 6 km wide and extrapolated to have contained up to 375,000 hawks (Fox 1956). On 2 Oct 1991, 5,014 Swainson’s Hawks were counted flying over Medina River, slightly farther west of San Antonio, in the hour before noon; that same morning, with a cold front, 15,000 flew over Mitchell Lake south of San Antonio; and a week later 10,000 passed northwest of Medina River (Fox 1956, Economidy 1992b). In e. Mexico, even larger concentrations passed just west of Gulf of Mexico and over Veracruz (Ruelas Inzunza et al. 1996; see Demography and populations: population status). Between Mexico and Panama, most transients occur on Pacific slope (Howell and Webb 1995). They usually fly over Nicaragua, for which there are no specimen records, without stopping (T. Howell pers. comm.). Large numbers are counted at Talamanca, Costa Rica at a newly recognized migration site, second only to Cardel in Vera Cruz, Mexico (Porras-Penaranda et al. 2004). They cross the upper Andes in Colombia and travel along eastern foothills of Andes, south through w. Brazil and e. Bolivia to Argentina. As birds enter n. South America and disperse over larger area, size of migrating flocks decreases (Hilty and Brown 1986).

Observations in e. North America have increased since 1970s, particularly in Ontario (12 fall and 5 spring records; Duncan 1986), Pennsylvania (3 in 1995; Hohenleitner 1995), Massachusetts (2 in 1988; Veit and Petersen 1993), and New Jersey (≤7 sightings/yr at Cape May alone; Sibley 1993). Still considered very rare along East Coast in fall. First mention in Canadian maritime provinces occurred in Nova Scotia in 1964 (Tufts 1986). The first Nova Scotia specimen, carrying a band applied at Cape May, New Jersey the previous September, was found dead in spring 1989 at Advocate Harbour, Nova Scotia (Houston and Schmutz 1995). The first fall sighting in Newfoundland was on 5 Oct 1998 (Mactavish 1999) and the first spring sighting was on 8 May 1999 (Maybank 1999). Other first sightings on the East Coast included New Brunswick 10-11 Oct 2002 (Mactavish 2003), North Carolina 3 Oct 1993 (Dean and Dinsmore 1997), and Long Island 18 Sept-4 Oct 1998 (DiCostanzo and Hays 1999). Status of these birds is unknown; they could winter in Florida where a small number arrive in Oct (Stevenson and Anderson 1994). The increase in eastern sightings may reflect an increase in birdwatchers and raptor enthusiasts, but increasing observations at some eastern hawk watches that have been carefully monitored for years suggests a possible long-term trend.

Sightings in the Caribbean were first reported in the mid-1990s on islands as far south as Trinidad (Hayes 2001).

Band recoveries and satellite telemetry data indicate central Argentina to be the main austral ground but there are also sightings in Brazil and museum specimens collected in Chile in 1875 and 1923 (Marin 2000). Banded birds, with adults over-represented from ne. California, Saskatchewan, and Colorado co-occurred in e. La Pampa Province, Argentina, in Jan 1994 (Woodbridge et al. 1995b); in 1994–1995, birds with satellite transmitters from ne. California, Idaho, Alberta, Saskatchewan, and Colorado co-occurred in same region (MJB). These results suggest that flocks in South America consist of individuals from different regions of breeding range (Woodbridge et al. 1995b). Geographic origins of overwintering populations in California and Florida unknown.

Spring

Large concentrations in Argentina begin to disperse in mid-Feb and begin moving north in late Feb–mid-Mar (MJB, B. Woodbridge pers. comm.). Transients recorded Feb–Mar in Colombia (Hilty and Brown 1986); Mar–early Apr (occasionally late Feb–late Apr) in Panama (Ridgely and Gwynne 1989); Mar–Apr in Mexico (Howell and Webb 1995), typically peaking in Veracruz, Mexico, 10–13 Apr (Ruelas Inzunza et al. 1996); mid-Mar–May (occasionally early Mar), peaking first half of Apr, in s. California (Garrett and Dunn 1981); peak late Apr–early May (earliest date 7 Mar) in British Columbia (Campbell et al. 1990); early Apr (earliest 14 Mar), peaking mid–late Apr, in Missouri (Robbins and Easterla 1992); and peak late Apr (recorded late Mar–mid-May) in Minnesota (Janssen 1987). Most generally return to Alberta and Saskatchewan between late Apr and 10 May (CSH).

Swainson’s hawks’ northward migration largely retraced their southward route. Averaging only 150 km/day (n = 17 complete northward migration tracks), it took them 60 days to complete the shorter 11,952 km migration back to North America (Fuller et al. 1998, Bechard et al. 2006).

Migratory Behavior

Migration is diurnal and timed to take advantage of rising thermals of hot air. In s. Texas, “the spectacular flights of fall migrants are, on sunny days, usually high in the air, where birds soar along, seldom flapping. They pass in long straggling lines or in clusters, which sometimes pause to wheel even higher on thermal updrafts” (Oberholser 1974: 234). May move in large flocks because thermal energy occurs in patches, zones, or waves (Smith 1985a). Hawks that were followed in a glider at 700 m entered early stages of developing thermal “streets” (i.e., lenticular zones of constant thermal uplift), then glided, largely out of sight from the ground, in lower 3 m of long, flat clouds (Smith 1985a). A hawk entering such a “street” may sail 60 km or more in straight line without losing altitude (Smith 1985b). Can also exploit lift along face of storm fronts, and slope- and wave-soar up and into the dry-season inversion zone to altitudes exceeding 6,000 m (Smith 1985a). Rarely fly over ocean, but in 1982, after unusually persistent northeast trade winds blew them onto the Azeuro Peninsula of Panama, they flew 30 km over Bay of Panama (Smith 1985a). At night, will roost in flocks of >100 and begin migration the following morning when thermals form (Smith 1980). During northward migration, late Feb–Apr, individuals tack in and out of mountains against and with northeast trade winds, often achieving very high speeds (Smith 1985a).

Control And Physiology

No information available on hormonal control of migration or physiological changes. Species is almost unique in switching from diet of primarily small mammals when raising young to insects when nonbreeding birds collect in midsummer flocks (Johnson et al. 1987), or when adults and immatures prepare to migrate.

Body fat increases prior to migration (Smith et al. 1986), but controversy persists about how far and how long Swainson’s Hawks can travel using these fat stores without food (Goldstein and Smith 1991, Kirkley 1991). However, the fasting migration model has had little empirical support based on body masses of hawks during migration (Bechard et al. 2006). There are occasional places en route through Central America and n. South America where they can forage; e.g., one account documents Swainson’s Hawks eating caterpillars on the ground in Costa Rica (P. Slud in Smith 1980). However, Skutch failed to see any sign of excretion below roosts of migrating Swainson’s Hawks; this was in keeping with his failure to see any sign of eating or hunting as the birds passed over El General, Costa Rica. (Skutch 1998). Likewise, where thousands of migrants roosted overnight in Panama there were no excreta, suggesting fasting birds at that location (Smith 1980).

Migration from Mexico to Argentina can be covered in approximately 3 wk (see Migration: timing and routes of migration); given that some suitable foraging habitat does occur in Central and n. South America, extended periods of fasting may not be common. Report that some individuals arrive in Argentina so weak they can be picked up by hand (C. C. Olrog in Smith 1980) could be due to fasting, or alternatively could be result of birds that encounter bad weather which extends the migration period and prevents foraging, or possibly birds that encounter pesticides en route to Argentina (B. Woodbridge pers. comm.).

An assessment of nutritional condition of free ranging hawks in Cordoba province, Argentina, revealed no statistical differences among sex and age groups for urea, uric acid and triglyceride serum concentration. However, cholesterol concentration was higher in male (2999 mg/liter) than female hawks (2167 mg/liter). (Sarasola et al. 2004).

Feeding

Main Foods Taken

Vertebrates, including mammals, birds, and reptiles (Schmutz et al. 1980, Bednarz 1988); invertebrates (especially grasshoppers, dragonflies and lepidoptera larvae) at other times (McAtee 1935, Sherrod 1978, Jaramillo 1993, Goldstein et al. 2000, Canavelli et al. 2001, Sarasola and Negro 2005).

Microhabitat For Foraging

Forages in open grassland, shrub steppe, and agricultural areas in North American breeding range. Often forages exclusively in row, grain, and hay crop agriculture. Exploitation of prey maximized by farming operations such as disking, seeding, cultivating, swathing, and baling. Flood irrigation of alfalfa fields and burning of fields concentrates foraging at edges of fields (J. A. Estep pers. comm.). On wintering grounds, apparently eats exclusively insects such as grasshoppers (Dichroplus spp.), dragonflies (Aeshna bonariensi), butterflies and moths (Lepidoptera), and leaf beetles (Coleoptera: Chrysomelidae) found abundantly in alfalfa fields and crops such as sunflowers and corn (White et al. 1989, Jaramillo 1993, Woodbridge et al. 1995b, Goldstein et al. 1996, Serracin Araujo and Tiranti 1996, Goldstein et al. 2000, Canavelli et al. 2001, Sarasola and Negro 2005).

Food Capture And Consumption

During the breeding season, a soaring, open-country hunter. Sometimes hunts high in air (Bent 1937), but more frequently courses low over prairie (Palmer 1988). Rarely observed flying low at high speed as Ferruginous Hawk does (Schmutz et al. 1980). Often hunts from perches such as tree limbs, poles or posts, rocks, and elevated ground. Like other hawks, follows farm equipment ranging from horse-drawn implements to tractors and pesticide applicators to prey on rodents disturbed by these activities (Preston 1885, Clark and Wheeler 1987, Estep 1989, MJB). Groups of Swainson’s Hawks will perch on the ground near ground squirrel (Spermophilus spp.) holes waiting for them to emerge, especially near dusk (Fisher 1893). When hunting pocket gophers (Thomomys spp.), perches near fresh mounds, waiting for gophers to push fresh dirt to surface; then pounces stiff-legged on mound and pulls out the gopher (MJB).

Nonbreeders hunt communally and eat wide variety of prey, from bats to flying insects. Can walk easily and run expertly; several accounts describe them pouncing at and running down grasshoppers and crickets in large groups like domestic turkeys do (Coues 1878, Fisher 1893, Bent 1937, Johnson et al. 1987). Catches flying insects in midair with talons and eats them in flight (May 1935, Bent 1937, Woodbridge et al. 1995b); may consume aerially caught insects at rate of up to 6/min (Woodbridge 1991). Described feeding on free-tailed bats (Tadarida spp.) in Oklahoma by snatching them from a stream of bats; 1 hawk caught a bat, transferred it to its beak, then caught another in its talons and flew off (Harden 1972). Only one study on hunting behavior in agricultural lands of Argentina (Sarasola and Negro 2005). Ground and aerial foraging are the main hunting methods employed by hawks in agriculture lands of Argentina either by walking on ground or by catching prey in their talons in air (White et al. 1989, Jaramillo 1993, Rudolph and Fisher 1993, Woodbridge et al. 1995b, Sarasola and Negro 2005). However, Swainson’s Hawks are most successful when hunting insects while soaring in mid-air (65% of prey capture attempts) than while walking on the ground (42%) (Sarasola and Negro 2005)

In central California, nesting pairs vigorously defend area surrounding nest, but away from nesting territory hunts communally in fields being harvested, disked, irrigated, or burned (Estep 1989). Then returns prey to nests. Foraging groups sometimes form in fields adjacent to nest sites during harvesting, even though fields previously defended by nesting pairs.

Diet

Major Food Items

Major rodent prey during breeding season include ground squirrels, pocket gophers, voles (Microtus spp.), and deer mice (Peromyscus spp.; Fisher 1893, Bent 1937). In n. Great Plains of w. Canada, Richardson’s ground squirrel is major prey species. West of Rocky Mtns., routinely eats rabbits in Utah and New Mexico (Smith and Murphy 1973, Bednarz 1988); in California, voles are more frequent part of diet (Estep 1989). In s. Alberta, Richardson’s ground squirrels averaged 89% (86-92%) of prey items in nests; of these, 97% were young ground squirrels despite the fact that adults made up 44% of populations in adjacent areas (Schmutz and Hungle 1989). Near Hanna, Alberta, Burrowing Owls (BUOW) were a major prey item where 45% of adult BUOWs were taken by Swainson’s Hawks (Clayton and Schmutz 1999). In Washington, frequently eats pocket gophers and snakes (Bechard 1980, Fitzner 1980); in Arizona and New Mexico, lizards and snakes (Bednarz 1988). Numerically, insects can account for as much as 90% of diet, but they are far less important on biomass basis (Snyder and Wiley 1976, Estep 1989). Insects are only small portion of diet of breeding birds (Smith and Murphy 1973, Dunkle 1977, Fitzner 1978, Bechard 1980, Bednarz 1988).

Diet of nonbreeders in North and South America is dominated by insects (Snyder and Wiley 1976, Johnson et al. 1987, Jaramillo 1993, Rudolph and Fisher 1993, Serracin Araujo and Tiranti 1996, (Sarasola and Negro 2005). In North America, flocks of nonbreeding hawks can feed exclusively on insects. Invertebrates made up as much as 94% of 3,428 food items recovered from stomachs of nonbreeding Swainson’s Hawks collected in North America (Snyder and Wiley 1976), and 1 Swainson’s Hawk collected in Kansas had 98 crickets (Acheta spp.) in its crop and 132 in its stomach (White 1966). Pellet analysis has shown that a single hawk can consume an average of 100 grasshoppers/d (Johnson et al. 1987). In South America, flocks of as many as 12,000 hawks also feed almost exclusively on insects, especially grasshoppers while foraging on ground (Woodbridge et al. 1995b, Goldstein et al. 1996). Aerial-hunting on grasshoppers results when grasshoppers and hawks use the same thermals. (Sarasola and Negro 2005). Stomachs of dead hawks have been found to contain only insects (Serracin Araujo and Tiranti 1996, MJB), with as many as 40–50 grasshoppers in each stomach (Zotta 1931). Excluding 1 pellet containing rodent jaws, >400 pellets analyzed have contained grasshopper and beetle remains and grass.

Quantitative Analysis

Dietary composition varies among regions (Table 1). Richardson’s ground squirrels are key part of diet in Alberta, Saskatchewan, and N. Dakota (Schmutz et al. 1980, Gilmer and Stewart 1984, CSH). In Wyoming, 68% of vertebrates eaten are mammals, and up to 25% are birds (Dunkle 1977). In Washington, diet is 25% mammals, 20% reptiles, and 19% birds (Fitzner 1980); in Utah, >50% of diet is rabbits (Smith and Murphy 1973); in ne. California, diet is chiefly small rodents and birds (Woodbridge 1987); and in New Mexico, rabbits, reptiles, and insects make up nearly 85% of diet. No quantitative information on diet during migration. In Argentina, regional differences in diet composition probably are linked to local insect abundance. In Buenos Aires province, >90% of Swainson Hawk diet was dragonflies Jaramillo 1993; in La Pampa, grasshoppers (>95%, mainly Dichroplus sp.) and lepidoptera larvae (>60%). (Serracin Araujo and Tiranti 1996, Canavelli et al. 2001, Sarasola and Negro 2005).

Food Selection And Storage

Generally an opportunistic predator, focusing on prey that is most available. Food-caching not reported.

Nutrition And Energetics

Nutrition poorly studied. Individuals apparently increase food intake following nesting and prior to migration in order to build up body fat reserves; such fattening may be necessary for migrating long distances without eating (Smith et al. 1986, Goldstein and Smith 1991, Kirkley 1991). Evidence concerning the extent of such fat deposition is still inadequate; dozens necropsied in Argentina in Jan had noticeable fat deposits (MJB).

Metabolism And Temperature Regulation

Metabolism not studied. At low ambient temperature, “fluffs” plumage and retracts 1 leg into feathers; at high temperatures seeks shade and becomes inactive, and gular flutters (CSH, ASE).

Drinking, Pellet-Casting, And Defecation

Drinks in captivity (Johnson et al. 1987, MJB). Generally, casts 1 pellet/d. No information on defecation rates.

Vocalizations

Development

On basis of female raised in captivity, first vocalizations similar to those of a kitten. At 7 wk, voice loud and shrill like that of a gull, but more piercing; at 8 wk, nestling made insistently repeated call of 4 notes; after 12 wk, gave very soft low whistle, but screamed when guarding food (Cameron 1913). Nestlings and postfledging birds frequently give begging call whenever they see adults carrying prey; also direct begging calls at siblings and at adult hawks of other species (Fitzner 1978). Younger chick usually more vocal than older sibling when adult delivers food (Porton 1977).

Vocal Array

Adults reported to give 3 different calls (Fitzner 1978). Characteristic Adult Scream, or Alarm Call (Fig. 4), commonly given by both sexes either in flight or from a perch; described as a shrill, rather plaintive, kreeeee (Taylor and Shaw 1927), a plaintive whistled kree-e-e (Bent 1937), or a high-pitched keeeoooooeee or keeeoooo fading off toward the end (Fitzner 1978). This call is given by both sexes in response to intra- and interspecific intruders, including humans near nest, and by female in response to male at nest or delivering prey (Porton 1977, Fitzner 1978). Female’s call shorter and lower-pitched than male’s Adult Scream (Dunkle 1977). No geographic variation reported.

Agonistic Pursuit Call given during boundary disputes; qualities of call not described by Fitzner (1978), but call is consistent with repeated pi-tick pi-tick call described by Bendire (1892).

Solicitation Call is a soft, 1-syllable weeeeee... given during copulation, presumably by female (Fitzner 1978).

Phenology

Reported as silent outside of breeding season (Palmer 1988). When flushed from roosts in Argentina, some individuals will give call similar to Adult Scream given during breeding season (MJB).

Daily Pattern Of Vocalizing

May call any time of day during breeding season, but calling most common early to late morning (ASE).

Places Of Vocalizing

Will vocalize in flight or when perched, including from nest or nest tree when provisioning young and calling to mate (Porton 1977, ASE).

Social Context And Presumed Functions Of Vocalizations

See descriptions of calls above.

Nonvocal Sounds

None reported.

Phenology

Pair Formation

Initiation of pair-bonding begins upon return to breeding grounds, Apr in colder part of range but early Mar in central California. Aerial courtship displays (see Behavior: sexual behavior, above) performed with decreasing intensity and frequency as breeding season progresses. No information on possible initiation of pair bonds on wintering grounds or during migration, and no documented interchange of banded birds between regional populations.

Nest-Building

Begins within 7–15 d of arrival and usually lasts about 1 wk. Most nest-building occurs during first 4 h of daylight (Fitzner 1980).

First/Only Brood Per Season

Figure 5. Egg-laying through fledging lasted about 73 d in a single nest and 99–110 d for the entire population (n = 36; Olendorff 1973). In se. Washington, clutches initiated within 7–14 d of arrival on breeding grounds and in first 4 h of morning (Fitzner 1980). Initiation dates vary latitudinally and locally with little synchrony. Earliest laying dates (early Mar–late Apr) occur in Arizona, New Mexico, Oklahoma, and Texas (Bent 1937, Bednarz 1988). Most clutches completed by mid-Apr in Central Valley, CA (Estep 1989). Initiation dates in ne. California, Oregon, Washington, Kansas, and Colorado are variable (late Apr–late May), with differences of up to 25 d occurring between adjacent pairs (Woodbridge 1987). In British Columbia and Saskatchewan, clutches are started from late May through mid-Jun (Bent 1937, CSH). Eggs hatch 34–35 d later. Nestlings fledge on average at 43 d of age (range 38–46; Olendorff 1973, Fitzner 1980, Woodbridge 1987). Fledging date averages 18 Aug in ne. California (Woodbridge 1987), 1 Jul–mid-Aug in central California (Estep 1989), and mid-Jul–mid-Aug in Colorado (Olendorff 1973). In Washington, 15 Jul reported to be earliest fledging date; 25 Jul–5 Aug more typical (Fitzner 1980). Most nestlings fledge during first 2 wk of Aug near Kindersley, Saskatchewan (CSH).

Nest Site

Selection Process

Both members of pair participate in nest initiation. Male selects nest site; not known if female plays role in selection. Once the site is determined, nest-building is usually completed in about 1 wk but occasionally takes ≤2 wk (Fitzner 1976).

Microhabitat

No information.

Site Characteristics

Typically a solitary tree, bush, small grove, or line of trees along stream course. Typical nest trees include willows, black locusts (Robinia pseudoacacia), box elders (Acer negundo), junipers (Juniperus spp.), oaks (Quercus spp.), aspens, and cottonwoods. In Cimarron Co., OK, 71% of nests were in Siberian elms planted around homesteads (McConnell et al. 2008). Small number of nests reported on human-built structures such as power poles or transmission towers (Olendorff et al. 1981, James 1992). Nest appears more flimsy or ragged than that of other buteos; can be any height but is usually near top of tree, within crown on small limb. Sites frequently attributable to human activity, such as shelterbelts (Olendorff 1973). Favors agricultural areas, including irrigated alfalfa fields (Woodbridge 1987), wheat fields (Bechard et al. 1990), and fields planted with row crops (Estep 1989, England et al. 1995).

In the Regina plain, Saskatchewan, nests were surrounded significantly more by grassland and trees and less wheatland (Groskorth 1995). In N. Dakota, pasture and haylands are predominant type of land within 1 km of occupied nests (Gilmer and Stewart 1984). In extreme s. Arizona, 41 nests in mesquite (Prosopis spp.) bushes 1–5 m above ground (Bendire 1892). Urban pairs in central California prefer conifers; 79% in Davis (n = 14) and 94% in Stockton (n = 17) (England et al. 1995). Nest trees in se. Washington significantly closer to human habitations than nest trees of Ferruginous Hawks (mean 4,806 m ± 713 SE vs. 2,524 m ± 383 SE, p <0.001; Bechard et al. 1990). In 1997, 27 nests were found on decommissioned telephone-line poles on the White Sands Missile Range in sw. New Mexico, usually where two vertical poles supported four paired sets of cross arms (Brubaker et al. 2003).

Nest

Construction Process

Both members of pair build or refurbish nest. In Washington, nest construction begins 7–15 d after arrival and requires 1–2 wk to complete (Fitzner 1980). Male brings most nesting materials and does most construction. Both sexes bring green sprigs.

Structure And Composition Matter

Typical raptor nest: bulky, unsightly mass of sticks. Constructed of various freshly broken sticks, twigs, and debris (<1 cm diameter). Sometimes baling wire, rope, and pieces of farm equipment incorporated, but less frequently than in nests of Ferruginous Hawks (CSH). Lining consists of fresh leafy twigs from nest tree, grass or hay, weed stalks and bark, but rarely cow dung (Bent 1937, Fitzner 1978, Palmer 1988). Nests in Montana consisted of sticks combined with materials such as sagebrush (Artemisia spp.), wild rose (Rosa spp.) brambles, and cottonwood twigs, with elaborate linings consisting of fresh leaves, green weeds, and wool (Cameron 1913). Frequently uses Russian thistle (Salsola kali) in nests (Fitzner 1980), which results in frail nest susceptible to damage by high winds (CSH). In se. Alberta, nests contained leaves (50%), forbs (40%), dung (<1%), and sod (10%), but none contained bark, as did nests of Ferruginous and Red-tailed hawks (Schmutz et al. 1980).

Dimensions

Outside diameter usually about 60 cm; inner bowl up to 20 cm wide and 6–7 cm deep (Palmer 1988). Mean nest depth 32 cm (range 10–122, n = 89; Schmutz et al. 1980). Depth of bowl decreases as flattened by nestlings.

Microclimate

Semiexposed; nests below tree canopy. Tends to nest toward top of tree and toward tips of smaller branches, making nests vulnerable to windthrow and rain.

Maintenance Or Reuse Of Nests, Alternate Nests; Nonbreeding Nests

Some nests used for ≥1 yr by same pair. More than 50% of nests in Washington, N. Dakota, and Saskatchewan were freshly built; remainder were refurbished nests of Swainson’s Hawk, Black-billed Magpie (Pica pica), American Crow, and Common Raven (Fitzner 1978, Gilmer and Stewart 1984, CSH). Unlike Red-tailed and Ferruginous hawks, which refurbish several nests in a particular year, Swainson’s Hawk does not build >1 nest/yr, but if the first clutch is destroyed or the nest blown down, a new nest may be built nearby (Sharp 1902, D. Zazelenchuk, pers. comm.).

Eggs

Shape

Short subelliptical to elliptical (Bent 1937, Harrison 1979).

Size

Length x breadth (mm): 57.1 (range 51.5–60.0) x 44.4 (range 41.2–46.7), n = 54 eggs in 20 clutches; Western Foundation of Vertebrate Zoology ). Similar averages reported by Reed (1904), Bent (1937), Schonwetter (1961), and Bechard and Houston (1984).

Mass

Average weight at various stages of incubation 54.9 g (n = 7; Olendorff 1973).

Color

Approximately 20% plain (off-white) or nearly so; others irregularly or sparsely marked with dark reddish brown or pale purplish blotches around larger end (Reed 1904, Harrison 1979, Palmer 1988).

Surface Texture

Smooth or finely granulated (Palmer 1988).

Eggshell Thickness

From WFVZ (n = 20 clutches, 54 eggs): pre-1950 mean empty shell weight 5.37 g (range 4.79–5.90); mean shell thickness 0.400 mm (range 0.370–0.437). In California, pre-1945 mean eggshell thickness 0.402 mm ± 0.032 SD (n = 50); 1979–1983 thickness 0.385 mm ± 0.028 SD (n = 14; Risebrough et al. 1989). In Columbia River Basin of Oregon, pre-1947 mean eggshell thickness 0.428 mm ± 0.005 SE (n = 31); 1976–1980 thickness 0.387 mm ± 0.007 SE (n = 35; Henny and Kaiser 1979, Henny et al. 1984). This 4% thinning had no measurable impact on breeding success. Low amounts of eggshell thinning correlate with predominantly insect diet in raptors (Newton 1979). For more detailed discussion, see Conservation and management: effects of human activity, below.

Clutch Size

See Demography and populations: measures of breeding activity: clutch, below.

Egg-Laying

Occurs at approximately 2-d intervals, with laying usually in morning hours (Fitzner 1980). In Wyoming, no evidence of re-laying after clutches destroyed (Dunkle 1977), but in Colorado, 1 instance of re-laying 14–16 d after a nest was destroyed (Olendorff 1973). No data on how likelihood of re-laying varies with nest phenology at time of nest destruction.

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

Begins with first egg laid.

Incubation Period

Initially estimated to be 28 d (Bent 1937, Brown and Amadon 1968), but now known to be 34–35 d (Olendorff 1973, Fitzner 1980).

Parental Behavior

Female does nearly all incubation; male covers eggs only while female feeds away from nest for brief periods during day. In Washington, female stays in nest during most of incubation period, and male provides her with food. During food transfers, male sometimes brings food directly to nest, but female typically flies to intercept prey from male; then consumes it away from nest. During absence of female, male assumes same incubation posture but keeps lower profile on nest, resting head on edge of nest. Female generally returns to nest within 10 min, but absences of 30 min are not uncommon. Female absences also associated with preening, collection of nesting material, and defecation. Neither male nor female observed defecating from nest; instead, they fly to perches >100 m away to defecate (Fitzner 1980).

Hatching

Takes 2–4 d (Fitzner 1980); pipping to hatching lasts 1–2 d. At two 3-egg nests, all eggs were laid at approximately 2-d intervals but the last egg hatched 1 and 3 d after the first egg; at a 4-egg nest the first egg was laid 1 d before the second, the subsequent eggs were laid at 2-d intervals but the last egg hatched only 3 d after the first (Fitzner 1978).

Young Birds

Condition At Hatching

Altricial and nidicolous. Hatchlings unable to raise head; lie limp for first few hours after hatching. Average weight at hatching is 39.4 g (Olendorff 1974b).

Growth And Development

Young inactive for first 8–10 d (Fitzner 1980); all activity during this period associated with feeding. Able to stand on feet at 13–17 d of age; prior to this, crawl on tarsi. First signs of sibling aggression at 27–30 d, when young begin tearing food apart themselves. Primaries first emerge at 9–11 d; tail-feathers at 14–15 d (Fitzner 1980). Gain in body mass is sigmoidal; growth of 7th primary is near linear (Parker 1976). Details on growth of young provided in Olendorff 1974b, Parker 1976, and Bechard 1983 .

Fratricide

Possible and confirmed records of fratricide (Parker 1976, 1979, Pilz and Seibert 1978). In 4-yr study in se. Washington, high nestling mortality occurred when young were 15–30 d old. Not always evident whether deaths of nestlings resulted from starvation or from fratricide, but bloody heads of live birds indicated the latter to be common cause of death. Partially eaten young, always the youngest or youngest 2 nestlings in brood of 3, found at 10 of 16 nests in 1978 and 15 of 26 nests in 1979 (Bechard 1983). Fratricide may be related to food availability, but ultimate cause remains unknown (Bechard 1983).

Parental Care

Brooding

From Fitzner 1980, se. Washington. Mostly by female, who broods during daylight hours for about 9 d. On first day after hatching, broods 78% of daylight hours, <10% on day 9. Female broods at night until young are 17–22 d old. At this age, she continues nest attentiveness, but behavior shifts from brooding to shading nestlings, especially on hot, sunny days.

Feeding

Male provides most of the food for female and brood, but female hunts more frequently as nestlings grow. Adults pick apart prey brought to nest and present it piece by piece to young; each feeding takes about 10 min and seldom exceeds 15 min (n = 3 nests observed; Fitzner 1980). Feeding frequency peaks at 10–15 d, gradually decreases prior to fledging (Fitzner 1980). Young first feed themselves at 23–26 d of age, but female periodically feeds young until 27–32 d old. When prey are scarce and nestlings only 2–4 wk old, adults may hunt at such a distance that neither one responds to humans when nests are disturbed (Houston and Schmutz 1995a).

Nest Sanitation

Young usually eject feces over edge of nest. Adults add fresh vegetation to nest bowl during incubation and early stages of brood development (MJB).

Parental Carrying Of Young

Not reported.

Cooperative Breeding

Sometimes 3 individuals seen at nests, but mating status has not been determined. In ne. California, 1 subadult male acted as nest helper, provisioning young at nest attended by adult pair (Woodbridge et al. 1995a). Three adults at single nests in s. Alberta during 2 different breeding seasons may have been breeding pair and helper, or polygyny (Cash 1989).

In ne. California, polyandry observed in small proportion of territories monitored between 1984 and 1995 (B. Woodbridge pers. comm.). Both males copulated with female, contributed to nest-building, and provisioned young. Most polyandrous trios together only 1–2 yr, but 1 territory was occupied by a polyandrous trio for 8 consecutive years; during this time the female and 1 male were replaced by new adults.

Brood Parasitism

Not known to occur.

Fledgling Stage

Departure From The Nest

Young first exercise wings when 29–33 d old (Fitzner 1980). Nestlings venture onto limbs that support nests at 27–33 d. Young quickly adapt to branches and then spend little time on nest. First flight at 38–46 d. For first 7–10 d after first flight, young stay near nest and fly only to chase adults carrying prey. By 10 d after fledging, young can be as far as 1 km from nest.

Association With Parents Or Other Young

In se. Washington, juveniles associate with parent birds for average of 29.2 d (range 22–38 d) after fledging and remain within the adult territory during entire postfledging period, largely dependent on adults for food (n = 13; Fitzner 1980).

Ability To Get Around, Feed, And Care For Self

During first 2 wk after first flight, fledglings use range of <2 km2(Fitzner 1980); area affected by availability and homogeneity of perching structures. If perches are scarce, young simply use the few perches available and do not move much. In central California, 6 fledglings fitted with radio transmitters stayed within 1.0 km of nest for approximately 2 wk after first flight and were provisioned by adults (J. A. Estep pers. comm.). At 11–20 d after first flight, they left nesting territory, did not return, and could be 1.5–240 km from nest. During this time, fledglings can travel 80–160 km/d, typically forming groups of 8–30, but up to as many as 150 juveniles. Once they left nesting areas, they did not appear to be provisioned by adults.

Immature Stage

Little known; needs study. On departure for migration, adults and young separate and juveniles depart alone.

Measures Of Breeding Activity

Age At First Breeding; Intervals Between Breeding

On basis of color-banded individuals or birds with distinctive plumages, most birds apparently do not breed until ≥3 yr old (Cameron 1913, J. K. Schmutz pers. comm.); but in se. Alberta 2 females (but no males) have bred at 2 yr old (J. K. Schmutz pers. comm.). No information on age distribution of first breeding. In ne. California, 2 of 5 banded 3-yr-olds recaptured as territorial breeders and observed breeding for the first time were in subadult plumage (Woodbridge et al. 1995a). Capable of breeding annually, but no information on proportion of population that does so.

Clutch

Typically 1–4 eggs (Bent 1937, Brown and Amadon 1968); but successful 5-egg clutches recorded in Sakatchewan (Houston 1998) and sw. Idaho (McKinley and Mattox 2001). In se. Washington, most common clutch size was 2 (Fitzner 1980); in se. Washington from 1977 to 1979, average clutch size 2.66 eggs ± 0.47 SE (n = 33; Bechard 1983).

From 1970 to 1972 in ne. Colorado, average clutch size 2.34 eggs (range 1–4, n = 95), 41% 2-egg and 43% 3-egg clutches (Olendorff 1973). In se. New Mexico, 1981–1985, average clutch size 2.48 eggs ± 0.70 SD (n = 60; Bednarz and Hoffman 1986).

Unhatched Eggs

In Saskatchewan, 129 (6.3%) of eggs from 987 nests failed to hatch; 80 eggs were in nests with young (Houston et al. 1991). Bacteriologic analyses of 42 unhatched eggs, 5 with embryos, found Enterobacter spp. in 18 eggs, and E. coli, Pseudomonas, and Streptococcus in 12, 11, and 10 eggs, respectively (Houston et al. 1997).

Annual And Lifetime Reproductive Success

Table 3 . No information on lifetime reproductive success. Proportion of nesting attempts producing at least 1 fledgling varies greatly: in ne. California, mean 65.5% successful/yr ± 3.4 SE (range 47.9–80.0%, n = 11 yr; Woodbridge et al. 1995a); in rural habitat in central California, mean 81.8% successful/yr ± 3.9 SE (range 70.2–94.1%, n = 5 yr; England et al. 1995). In Saskatchewan, minimum annual nest failure rate (based on activity near hatching, excluding early nest losses) averaged 29.6% failed/yr (range 17.1–61.2%, n = 22 yr; Houston and Schmutz 1995a).

In Saskatchewan, number of young fledged/successful nest dropped from 2.09 in 1944–1987 to 1.60 in 1988–1994 (t = 5.73, df = 22, p < 0.0001); reproductive success declined simultaneously in Alberta. In grasslands, both trends coincided with declines in principal prey, Richardson’s ground squirrel. (Houston and Schmutz 1995a). From 1997-2004, hawks in western Saskatchewan raised an average of 1.9 young/nest (Houston and Zazelenchuk 2004). Annual adult survival in Saskatchewan and Alberta, 1972-2003, was 84.3%. Fidelity was 90%, indicating 10% of surviving adults permanently emigrated in any given year (Schmutz et al. 2006). In se. Washington, productivity dropped from 2.21 young/attempt in 1977 to 0.75 in 1978 and 1.12 in 1979; these decreases also coincided with a drop in prey availability (Bechard 1983). In ne. California, reproductive success was inversely correlated with distance to suitable foraging habitat (Woodbridge 1991). In central California, urban nesting birds were farther from suitable foraging habitat than were rural nests, and they fledged fewer young (England et al. 1995).

Number Of Broods Normally Reared Per Season

One.

Proportion Of Total Females That Rear Broods

No information available on proportion of breeding-age females attempting to nest; 1 and 2 yr old birds have not been observed nesting, and unknown proportion of 3-yr-olds do not attempt breeding (B. Woodbridge pers. comm.).

Life Span And Survivorship

Few data on annual survivorship rates. In ne. California, mean age for hawks banded as nestlings in 1980–1992 and observed in 1993–1994 was 8.2 yr ± 0.52 SE (n = 36; Woodbridge et al. 1995a). Longevity data for 410 birds banded as nestlings throughout North America: 86 birds survived ≥3 yr, and of these, 19.8% were ≥10 yr old when recovered (Houston and Schmutz 1995b).

Longevity records of 2 birds banded in Colorado: one 18 yr 2 mo when found dead under a power line in Texas, the other 19 yr 6 mo when recovered as part of pesticide kill in Argentina (Houston and Schmutz 1995b, Woodbridge et al. 1995b).The oldest male in the banding records to date was a male banded by Peter Bloom in California and retrapped by Brian Woodbridge 24 years later; the longest-lived female was at least 21 years old, having been banded at age two. Survival decreased, coincident with monocrotophos use in Argentina, from 1979 through 1996 (Briggs 2007). Three color-banded birds of known age at Hanna, Alberta, were observed at 16, 17, and 19 yr old respectively and 1 bird banded as an adult was ≥19 yr old (J. K. Schmutz pers. comm.). A 24-year-old returned to its breeding site in California where it was trapped in 2004 (B. Woodbridge, pers. comm).

Briggs (2007) found survival and reproductive success of hawks in the Butte Valley, nw. California decreased after mass die-offs in Argentina.

Disease And Body Parasites

Nestlings in 20–40% of nests in Saskatchewan and Alberta are infected with Haemoproteus pallidus or Leucocytozoon toddi, the most common blood parasite (Portman and Schmutz 1999). Myiasis by bird nest screw worm flies (Protocalliphora avium) of auditory meatus, axilla, nape and neck, and anal area of both of 2 nestlings examined in N. Dakota (Tirrell 1978). The Coccidia Caryospora kansensis and Sarcocystis/Frenkelia spp. were reported in feces from 3 of 4 Swainson’s Hawks examined at rehabilitation center in Kansas (Upton et al. 1990). A search for spores of Bacillus anthracis in feces of three wintering individuals was negative (Saggese et al. 2007).

Causes Of Mortality

Most nestling deaths are due to starvation, sometimes mediated by fratricide (Pilz and Seibert 1978).

From 547 encounters of 538 individuals (adults) reported in banding files through 1992, 396 were found dead; of these, 123 were shot, 122 killed on highways, and 151 died of unknown causes (Houston and Schmutz, 1995b). For human-caused mortality, see Conservation and management: effects of human activity, below.

Exposure

Storms and winds destroy many nests; wind and hail averaged 34% (30–43%) of nest failures annually from 1977–1979 in N. Dakota (Gilmer and Stewart 1984), killed 60 migrating hawks in Oklahoma (Jones 1952), and contributed to only 50 young being produced from approximately 200 nests in se. Colorado in 1973 (Kingery 1974). Storm front forced flock of 8,000 birds to the ground in Panama on 21 Oct 1981; some flapped heavily, crashed through trees, and lay stunned or injured on ground (Smith 1985b). Massive mortality in non-breeding grounds also related to severe weather conditions. In La Pampa province, 113 hawks were found dead in a single roost after a hailstorm (Sarasola et al. 2005). From 14 recovered alive, only ten survived one week later, increasing the total number of affected hawks to 117. Heavy rainstorms and winds can blow hawks out of roost trees (Goldstein 1997)

Predation

In Wyoming, eggs destroyed by crows in 5 of 13 nests near crow nests, and some nestlings taken by Great Horned Owls (Dunkle 1977). Brood reduction substantial (see Breeding: young birds, above).

Range

Initial Dispersal From Natal Site

From 1984 to 1994 within Butte Valley of ne. California, dispersal distances from natal sites to subsequent breeding sites averaged 8.8 km ± 1.1 SE (range 0–18.1 km, n = 41), and did not differ between sexes or from random inter-territory distances (Woodbridge et al. 1995a). Estimates may be low because of lower survey effort outside Butte Valley. During study period, 1 bird banded as nestling in Butte Valley bred in Klamath Basin, 36.8 km away, and 2 birds hatched in Klamath Basin moved 10.2 and 7.9 km to breed in Butte Valley (Woodbridge et al. 1995a). In central California, 2 birds retrapped as adults were within 3.5 km of their natal nest (Estep 1989). Of 697 color-banded nestlings in Saskatchewan since 1978, none were relocated in study area despite careful observation, but 3 were found 190, 200, and 310 km, respectively, away at nests in Alberta, suggesting long dispersal distances for that population (Houston and Schmutz 1995b). In Alberta, 4 color-banded males bred within 3–10 km of their natal sites, while 7 females moved 6–320 km (J. K. Schmutz pers. comm).

Banded nestlings were not re-sighted or found dead in Saskatchewan when one year old, and only 4 were encountered at 2 yr. A total of 7 were encountered at 3 yr, 4 each at 4 and 5 yr, 2 each at 6 and 7 yr, 3 at 8 yr, and 1 each at 9, 16, and 18 yr. Of these 30 nestlings, mean natal dispersal distance in the breeding season was 66.7 km (33.1 km for the 24 found dead and 206.8 for the 6 resighted alive (Houston 2005).

Band recoveries exaggerate degree of natal philopatry through inclusion of visual sightings and recaptures at subsequent nest sites and underestimate philopatry if one includes birds reported in May, Aug, and first 10 d of Sep, when they may not be on territory. For Jun and Jul records of dead birds >4 yr old, mean distance from natal site was 31 km (range 0–95, n = 14; CSH).

Fidelity To Breeding Site And Winter Home Range

Individuals frequently use same nest or nest tree in successive breeding seasons or move only short distances within same territory (Fitzner 1980, J. A. Estep pers. comm.). In ne. California, for 25 inter-territory movements between 1984 and 1994, mean adult dispersal distance is 2.2 km ± 0.23 SE (range 0.97–6.3; n = 11 males, 14 females); typical were short moves between neighboring territories, significantly less than the mean nearest-neighbor distance, 3.7 km ± 0.87 SE (Woodbridge et al. 1995a). No information on fidelity to winter range in South America.

Home Range

Home range size of breeding adults varies greatly (Table 4). Larger home ranges found in areas with crop types unsuitable for foraging, such as mature grains and row crops, orchards, and vineyards (Bechard 1982, Estep 1989, Babcock 1995). Smallest home ranges reported at nest sites near alfalfa, fallow fields, and dry pasture (Bechard 1982, Estep 1989, Woodbridge 1991). Home ranges become larger seasonally, when crops mature, providing cover and making prey unavailable (Bechard 1982, Estep 1989, Woodbridge 1991, Babcock 1995). A radio-tagged individual foraged up to 30 km from nest site for several days, then remained within approximately 1.5 km of nest when nearby alfalfa fields were mowed (J. A. Estep pers. comm.). Size of male and female home ranges overlap, but largest belong to males and smallest to females, with smallest female territories up to an order of magnitude smaller than largest male territories in studies reporting this information (Fitzner 1978, Estep 1989, Andersen 1995, Babcock 1995). In La Pampa province, Argentina, activity areas for six radio-tagged individuals (wintering) ranged from 557.3 to 1955.3 km2 (Canavelli 2000).

Population Status

Numbers

No comprehensive estimate of population size on breeding grounds or in wintering areas in South America. Number of migrants counted over Panama City, Panama, in Oct and Nov: 175,644 in 1970; 344,409 in 1972; 299,718 in 1973; 207,115 in 1976 (Smith 1980). Fall migration counts over Veracruz: 466,810 in 1992; 272,780 in 1994; 448,000 in 1995; 845,465 in 1996 (Ruelas Inzunza et al. 1996, pers. comm.). Comparisons between years not meaningful, because of methodology used (Smith 1985b), and migrant counts should not be used as indicator of total population.

Available estimates cover only part of species range, but cumulative total appears low, given number of migrants observed in Panama and large flocks observed in South America. Estimates of breeding pairs in 1987: 400–800 in Oregon, 175 in Washington, 150 in Nevada, 1,000 in Wyoming, 1,000–3,000 in Montana, and 400–500 in Colorado (Harlow and Bloom 1989); in 1986, >3,000 in New Mexico and <300 in Oklahoma (Bednarz 1986); and 5,200 in N. Dakota in the 1960s (Stewart and Kantrud 1972). Increase in estimate of California population from 375 breeding pairs in 1979 (Bloom 1980) to 550 in 1988 is likely because of increased survey effort (Calif. Dept. of Fish and Game 1988). Population in w. Canada estimated at 20,000–50,000 pairs (Kirk et al. 1995); estimate of 7,182 nesting pairs in se. Alberta in 1987 (95% confidence interval = 5,961–8,405) and 3,879 nesting pairs in 1982 (95% confidence interval = 3,077–4,681), extrapolated from 76 41-km2 random study plots (Schmutz 1987).

Nesting densities highest in areas with either a mixture of native habitat and agriculture or a high diversity of irrigated crops: central California, mean 30.23 pairs/100 km2 (range 21.39–39.31, n = 5 yr; England et al. 1995); Kindersley, Saskatchewan, 18.0 pairs/100 km2 (n = 1 yr; CSH); and Los Medaños, New Mexico, up to 16.7 pairs/100 km2, mean 9.5 pairs/100 km2 (n = 3 yr; Bednarz et al. 1990). Nest density near Hanna, Alberta was 13-16 pairs/100km2 from 1975-1985 and 1989-1994; when ground squirrel populations were most dense, nest density rose to 22-24 pairs/100 km2 from 1986-1988. When ground numbers crashed in 1995-1996, nesting density fell to 6-7 pairs/100 km2 (Schmutz et al. 2001).

In ne. California, overall density of territories was 20 pairs/100 km2 (n = 12 yr), but varied from 5.7 pairs/100 km2 in irrigated pasture to 36.8 pairs/100 km2 in landscape dominated by alfalfa (Woodbridge et al. 1995a). Reported nesting densities much lower in landscapes dominated by sagebrush and juniper or by crop types unsuitable for foraging: Cache Valley, Utah, 10 pairs/100 km2 (Bosakowski et al. 2001); Hanford, WA, 1.31 pairs/100 km2 (range 1.0–1.5, n = 4 yr; Fitzner 1980); se. Colorado, 1.15 pairs/100 km2 (range 0.4–2.1, n = 5 yr; Andersen 1995); se. Washington, 0.4 pairs/100 km2 (range 0.4–0.5, n = 3 yr; Bechard 1983); se. Idaho, 0.3 pairs/100 km2 (range 0.2–0.3, n = 3 yr; Hansen and Flake 1995).

On non-breeding grounds, up to 12,000 hawks may be found in single nocturnal roost (Woodbridge et al. 1995b, M. Goldstein and B. Woodbridge pers. comm.). In e. La Pampa Province, Argentina, foraging flocks dominated by adult birds range from 50 to 1,000 birds, with some flocks containing > 4,000 hawks (Woodbridge et al. 1995b, A. Lanussé pers. comm.). In contrast, foraging flocks of 5,000–10,000 individuals in e. Buenos Aires Province are dominated by juveniles (Jaramillo 1993).

Trends

When Canadian prairie settlement was beginning in 1882, Ernest E. Thompson considered Swainson’s Hawk “abundant” as far east as Winnipeg, Manitoba; Elliott Coues in 1873 made similar observations along the U.S.-Canada boundary south of Manitoba (Thompson 1891, Houston 1980). Sharp drop in numbers after settlement began was due largely to persecution; most early settlers on prairies who kept domestic poultry called this species the “chicken hawk” and generally believed that “the only good hawk is a dead hawk.” In 1893, in Cypress Hills of sw. Saskatchewan, with only 3 ranchers in 500 km, at no time could one cast one’s eye upward without seeing this hawk hovering in the sky; 20 yr later, it was conspicuous by its absence (S. Pearse in Bradshaw 1915). Similarly in e. Montana, this species changed from being one of the most common birds in 1900 to greatly decreased numbers by 1912 (Cameron 1913). Coincidentally in Alberta, A. G. Wolley-Dod reported that “Swainson’s buzzards... are now practically extinct” (Hornaday 1913). In Okanagan Valley of British Columbia there were flocks of hundreds with a flight lasting 5 h in 1889 (Campbell et al. 1990); flocks of 300–400 in 1892 declined to “unusual numbers” between 1913 and 1915, until flocks of only 75 were seen in Jun 1925; scarce by 1947 (Cannings et al. 1987).

Absent from much of its historical breeding range in central and s. California, where overall population may have declined by >90% during 1900s (Bloom 1980). Population declines also reported in se. Oregon (Littlefield et al. 1984) and Nevada (Herron et al. 1985). In brush country and on Edwards Plateau, TX, numbers may have risen as human population diminished, mammals increased, and ranchers planted kleingrass (Panicum coloratum) and saltbush (Atriplex spp.; Economidy 1992a).

Since 1987, reproductive success in Saskatchewan and Alberta has dropped to lowest recorded in 27 yr, and during 1994, number of nests in the most intensively studied township was half the 1987 level (Houston and Schmutz 1995a). This decrease in productivity may be attributed to low prey numbers, plowing of grasslands, and gradual decay of trees on prairies (Houston and Schmutz 1995a, CSH). In Saskatchewan and N. Dakota, Red-tailed Hawk is gradually moving south, even into pasturelands, filling gaps left by vacating Swainson’s Hawks; little or no evidence, however, that Red-tailed Hawk is displacing Swainson’s Hawk (CSH). On Lostwood National Wildlife Refuge, N. Dakota, Swainson’s historically was most abundant hawk; no Red-tailed Hawks bred there until 1962–1963. By 1986–1987, however, as result of fire suppression, diminished grazing, increased vegetation height and density, and increasing number and area of aspen groves, Red-tailed Hawk outnumbered Swainson’s Hawk 2 to 1 (Murphy 1993).

Swainson’s Hawk considered abundant and stable in Idaho (MJB), Washington, Montana, and Colorado (Harlow and Bloom 1989).

Microsatellite primers have been developed for the species Hull et al. 2007. Application of this markers to genetic population studies revealed high genetic diversity, a recent reduction in effective population size of Swainson’s Hawks across all regions examined in North America and a lack of genetic structure throughout breeding populations Hull et al. 2008)

Population Regulation

Few data; relative importance of various sources of mortality and factors affecting reproductive success is poorly documented and needs study. As with many raptors, availability of prey and of nest sites are key factors regulating populations (Newton 1979). For example, decline of prey populations has coincided with decline in reproductive success of Swainson’s Hawk in Saskatchewan (Houston and Schmutz 1995a) and in Washington (Bechard 1983). Lack of suitable nesting trees may limit populations in some local areas (Olendorff and Stoddart 1974). In some parts of the Great Plains, nest trees are primarily the result of human activities (Gilmer and Stewart 1984); long-term loss of trees—such as in the grasslands of sw. Saskatchewan, where nest trees are located primarily around abandoned farm sites and are gradually dying from neglect, drought, biocide spraying of adjacent lands, excessive rubbing by cattle, and bulldozing—could lead to local population limitation and decline (CSH).

Interspecific competition between Swainson’s and Red-tailed hawks has been proposed as contributing to declines of the former (Bloom 1980, Janes 1984). However, local shifts from numerical dominance by Swainson’s Hawk to that by Red-tailed Hawk in some areas may result from habitat alteration that favors Red-tailed Hawks (see Population status, above). Also, in Central Valley, CA, Swainson’s Hawks can successfully dislodge incubating Red-tailed Hawks (J. A. Estep pers. comm.).

No information on mortality during migration. Hailstorms and pesticide poisoning (see Conservation and management: effects of human activity, below) in Argentina is the only quantified information on mortality on wintering grounds but there are at least two individual records of this hawk being shot on its wintering grounds (Harlow and Bloom 1989, J. Schmutz, pers. obs).

Effects Of Human Activity

Shooting

Even though it preys on agricultural pests, Swainson’s Hawk was historically considered a “varmint” by many ranchers and farmers, until at least the late 1930s. In e. Washington, this species was subject to “continual persecution” by sheep herders and so-called “varmint hunts” organized by certain sportsman’s organizations (Bowles and Decker 1934). Birds were shot on the nest in Saskatchewan as “a local custom” (Munro 1935). Migratory flocks were shot at nocturnal roosts; roughly 225 of approximately 1,000 birds were killed at night on 26 Sep 1938 in grove of trees in Merrick Co., NE (Swenk 1939).

Banding recoveries suggest that mortality resulting from shooting on breeding grounds has declined and may not currently be significant; 60 of 81 (74%) returns from birds banded prior to 1955 had been shot, but only 63 of 421 (15%) since 1955 (Houston and Schmutz 1995b). Little information on shooting in South America, but one report of local farmers shooting migrating birds by lamplight in Colombia (Bloom 1980, Bildstein et al. 1993) and at least two birds shot in Argentina (Serracin Araujo and Tiranti 1996, JHS pers. obs.). Of individuals banded since 1955, only 5.2% recovered in U.S. had been shot; 44 of 96 (45.8%) recovered south of Rio Grande had been shot, including 14 of 16 recoveries in Colombia (CSH).

Pesticides And Other Contaminants/Toxics

Eggshell thinning and organochlorine residues in eggs not reported at levels known to adversely impact reproduction in Washington and Oregon (Henny and Kaiser 1979, Bechard 1981, Henny et al. 1984), N. and S. Dakota (Stendell et al. 1988), or California (Risebrough et al. 1989). Chronic residue levels low, despite migration to South America, where a wide variety of chemicals are still used; these levels are low probably because Swainson’s Hawks feed primarily on insects, which are low in the food chain (White et al. 1989).

Acute toxicity from poisoning by organophosphate insecticides monocrotophos and dimethoate, used to control grasshopper outbreaks in alfalfa and sunflower fields, caused the death of nearly 6,000 Swainson’s Hawks in Argentina in 1995 and 1996. Overall, an estimated 20,000 were killed in Argentina by pesticide applications. Deaths resulted immediately after hawks were sprayed directly by pesticide applicators while they foraged in fields or within several days after they ate poisoned grasshoppers. Many hawks died within minutes of being sprayed, and their carcasses were found in adjacent fields (Woodbridge et al. 1995a, Goldstein et al. 1996, MJB). Twenty-four hawks died in another presumed poisoning incident in Córdoba province in 1996-97, but the causes of that mortality were not clearly identified (Goldstein et al. 2000, Hooper et al. 2002).

For further details, see Demography and Populations: causes of mortality, above; also Management: effectiveness of measures, below.

Ingestion Of Plastics, Lead, Etc.

Not reported.

Collisions With Stationary/Moving Structure Or Objects

Of 538 banding recoveries throughout the breeding, wintering, and migratory range of Swainson’s Hawks, 51 were hit by car, 2 hit by train, 71 found on highway, 13 electrocuted on power lines, and 3 caught in fences (Houston and Schmutz 1995b).

Degradation Of Habitat

Because of human-caused habitat changes, Swainson’s Hawks now commonly breed in areas of intensive agriculture. Alfalfa fields and other hay fields seem particularly well suited to this species. These fields often support large numbers of prey such as insects, ground squirrels, and pocket gophers, and prey populations are not routinely disturbed by annual plowing; in addition, fields are periodically mowed so vegetation height does not prevent foraging. Fields with annual crops may also be used for foraging prior to planting and after harvest.

Trees planted by farmers near farm sites often provide nesting or roosting substrates. As small farms have been absorbed by larger agribusinesses, and original homestead sites with shelterbelts and tree plantings have been lost, these changing land uses may have harmed the species by removing nest sites (Olendorff and Stoddart 1974). Loss of roosting trees may be a factor on South American wintering grounds as native habitats are converted to cropland. On breeding and wintering grounds, conversion of native habitat to woody perennial crops and urban development eliminates Swainson’s Hawk.

On wintering grounds, foraging Swainson’s Hawks select pasturelands that are not rotated on an annual basis (perennial pastures that remain planted during several years) over other land-use types (Canavelli et al. 2003, Sarasola and Negro 2005, Sarasola et al. 2008a). Ongoing intensification of agriculture practices on the Argentine pampas, and conversion of mixed, cattle raising-crop systems to exclusively crop production (especially soybeans), leads to a significant reduction in habitat suitable for wintering Swainson’s Hawks (Sarasola et al. 2007).

Disturbance At Nest And Roost Sites

In some areas, pairs may desert nests visited by humans during nest-building and incubation (Bent 1937, Houston 1974); in other areas, the species appears to be less susceptible to such early disturbance (J. C. Bednarz and J. A. Estep pers. comm.). Pairs rarely desert nests by the time young are large enough to band (J. A. Estep pers. comm., CSH, MJB). Generally tolerant of regular, ongoing human activities around nest sites in agricultural and urban landscapes (Estep 1989, England et al. 1995). May abandon nest in response to loud, irregular, unpredictable activities, as was the case near work crews constructing a 230-kV transmission line in se. Colorado (Stahlecker 1975). Some individuals capable of successfully fledging young at nest sites with loud but regular noises associated with railroad tracks (James 1992) and major city intersections (England et al. 1995).

Management

Conservation Status

Without exception, reports of early-nineteenth-century and twentieth-century ornithologists attest to relative abundance of Swainson’s Hawks throughout the w. U.S., including e. Washington (Bowles and Decker 1934), e. Oregon (Gabrielson and Jewett 1940), and California (Remsen 1978, Bloom 1980). Now reduced in numbers or distribution throughout its range and considered to be declining in Utah, Nevada, and Oregon; listed as Species of Special Concern in Utah, Nevada, Oregon, and Washington, and as Threatened in California (Littlefield et al. 1984, Herron et al. 1985, Harlow and Bloom 1989). Currently no federal status under Endangered Species Act.

In California, evidence from egg collections suggests that this population has been reduced by as much as 90% from its estimated historical levels (Bloom 1980). This severe population decline in the Central Valley of California is corroborated by microsatellite analyses of DNA which suggest that the decline has taken place over 68-75 generations, or about 200 yr, which corresponds with the time of European settlement (Hull et al. 2008).

Measures Proposed And Taken

Proposed conservation measures, including habitat conservation plans, usually focus on retention of some portion of existing foraging and nesting habitats while allowing other areas to be lost to urban development. As economic conversion of agricultural areas to commercial and residential real estate continues, impacts on Swainson’s Hawk populations should be monitored to determine population trends. Disturbance of breeding pairs and destruction of nest trees need to be prevented (Sharp 1986). Alternative, less toxic pesticides and grasshopper baits should be tested in Argentina (see below). Few, if any, of these conservation measures have been formally undertaken.

Effectiveness Of Measures

No effective measures to mitigate for loss or degradation of foraging habitat has been demonstrated. Because Swainson’s Hawks avoid using open-topped structures for nesting, artificial platforms do not appear feasible as a method to augment nesting populations. Nest platforms are routinely used by Ferruginous and Red-tailed hawks (Bednarz 1988), however, so building platforms appears to favor these species over Swainson’s Hawks (Howard and Hilliard 1980, Janes 1985, 1987). Tree plantings in areas where nest sites are limiting are a preferred, alternative nest management technique (Fitzner 1980). No single tree species is required; species ranging from riparian trees such as willows or cottonwoods to more xeric species such as locusts (Robinia spp.), box elders, and mesquite are suitable.

With regard to pesticides, efforts to exclude monocrotophos in the La Pampa region of Argentina, where hawk poisoning was most prevalent, resulted in the absence of further mortality in the next austral summer (Goldstein et al. 1999c). However, predictive maps of Swainson’s Hawk occurrence and abundance have shown a broad area of pampas in se. Argentine where the species may winter at a relatively high density. This area was previously overlooked as a core area and the impact of insecticides on Swainson’s hawk populations has never been assessed there (Sarasola et al. 2008). Future field-monitoring and conservation actions should be focused in these areas, taking into account information and educational programs on the correct use of agrochemical compounds by local landowners.

Although the organophosphate pesticide identified as responsible for the massive mortalities of Swainson’s Hawks in the past (monocrotophos, MCP; Goldstein et al., 1999a) was banned in Argentina in 1999 (Resolution no. 182/99 from SAGPYA/SENASA, Argentina), other highly toxic organophosphate compounds have replaced it (Goldstein et al. 1999b; Hooper et al. 1999, 2002). Furthermore, during 1996–97 one Swainson’s Hawk mortality incident (24 birds affected) was recorded in Cordoba province outside of the MCP exclusion zone delimited in northern La Pampa province, although samples were not adequate to determine the cause of mortality (Goldstein et al. 1999b, Hooper et al. 2002). Consequently, it is unclear if the absence of new cases of mortality being reported in the Argentine pampas is due to insecticide regulatory measures, the effectiveness and geographical extent of educational programs, or just to less favorable weather conditions for pest outbreaks.

Swainson's Hawks have 10 functional primaries, 15-16 secondaries (including three tertials), and 12 rectrices. Accipitrine hawks are diastataxic (see Bostwick and Brady 2002) indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. Wings are long and moderately pointed (wing morphology p8 > p7 > p6 > p5 > p9 > p4 ~ p10 > p3 and with p8-p10 notched and p7-p9 emarginated); tail is squared. Geographic variation in appearance and molt strategies unreported (see Systematics: Geographic Variation) although proportion of dark-morph to light-morph birds (see Plumages) may be higher in more-western breeding populations.

Molts

Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). Swainson's Hawk exhibits a Complex Basic Strategy (cf. Howell et al. 2003), including incomplete-to-complete prebasic molts and a limited (occasionally absent) preformative molt (Pyle 2005a), but no prealternate molts (Bent 1937; Palmer 1988; Schmutz 1992; Wheeler 2003a, 2003b; Pyle 2008; MJB; Fig. 5). Either the second or third prebasic molt can result in definitive plumage aspect (see Plumage).

Prejuvenal (First Prebasic) Molt

Complete, May-Jul, in the nest. Begins at 9–10 d old, when primaries and secondaries first visible (Fitzner 1978). Scapulars and primaries form conspicuous dark stripes against light-colored downy plumage at 12 d. At 14 d, rectrices, scapulars, and remiges show as dark patches against white downy plumage. Breast-patches begin to darken by 16 d. By 20 d, scapulars, wing-coverts, remiges, and rectrices form dark contrast against light-colored head and belly. Head still downy at 26 d; first signs of contour feathers on head at 28 d. At 30 d, head-feathers half grown and remiges and rectrices about 65% developed. By 34 d, contour and flight-feathers completely developed (Fitzner 1978).

Preformative Molt

Only recently recognized (Pyle 2005a); absent to limited, Sep-Mar(?), primarily on non-breeding grounds; probably can suspend for migrations. Can include up to 40% of body feathers but appears to be absent in some individuals (Pyle 2005a, 2008). No wing coverts or flight feathers replaced.

Second Prebasic Molt

Considered "First Prebasic Molt" by some previous authors (see Howell and Corben 2000 and Pyle 2005a for explanation of updated terminology). Incomplete to complete, Feb-Jan(?), possibly can commence on winter grounds in South America but more typically begins on summer grounds in North America, suspends for or can continue during migration, and completes on winter grounds. Includes most to all feathers. Remiges exhibit Staffelmauser (stepwise) sequence and pattern of flight-feather replacement (Stresemann and Stresemann 1966; see Definitive Prebasic Molt); outer 1-5 juvenal primaries (among p6-p10) and corresponding primary coverts, 1-7 juvenal secondaries (among s3-s4 and s6-s10), and/or 1-4 juvenal rectrices (among r2-r5) typically retained to commence Staffelmauser (Pyle 2005b, 2006); molt can be complete in ca. 15-20% of individuals.

Definitive Prebasic Molt

Incomplete to complete, May-Mar. Can commence during incubation and suspend for chick feeding in breeding birds (2005b), and can suspend for or continue during southbound migration, completing on non-breeding grounds (Goldstein et al. 1999, Bechard and Weidensaul 2005). Usually includes most to all body feathers and most flight feathers but can be complete in ca. 10% of individuals. Primaries replaced distally (p1 to p10; Miller 1941), secondaries replaced proximally from s1 and s5 and distally from the tertials, and rectrices possibly replaced in sequence r1-r6-r3-r4-r2-r5 on each side of tail, as in other Buteo hawks. Females and males show differences in remigial molt patterns (Schmutz 1992), perhaps related to sex-specific differences in replacement strategies during incubation, females typically replacing more feathers than males in most North American hawks (Pyle 2005b). Commencement of molt can be delayed during seasons of poorer or later breeding productivity, suggesting a nutritional effect (Schmutz 1992).

Molt pattern among primaries and secondaries can exhibit Staffelmauser (Stresemann and Stresemann 1966; Clark 2004; Pyle 2005b, 2006) whereby incomplete molts result in series of commencement points within primaries and secondaries, beginning with termination points of previous prebasic molt and also initiating new series commencing at p1, s1, s5, and/or the tertials. Replacement thus typically proceeds in 2-4 (less commonly 1 and rarely 5) waves through the wing, resulting in 1-4 "sets" (generations) of feathers following completion of molt. Staffelmauser appears to be a product of insufficient time to undergo a complete wing-feather molt but has adaptive benefits in producing multiple small gaps in the wing during molt, which retains wing integrity and ability to fly and forage (Tucker 1991, Shugart and Rohwer 1996, Pyle 2005b). Staffelmauser can result in substantial individual variability in molt patterns and asynchrony in replacement positions between left and right wings (Schmutz 1992).

Plumages

Swainson's Hawk exhibits polymorphism (Palmer 1988). Morphs often grouped in 3 categories based primarily on coloration of underparts (light to dark, and rufous; Palmer 1988, Wheeler and Clark 1995). However, variation is almost continuous from lightest to darkest individuals, making the morph categories convenient but somewhat arbitrary constructs (Palmer 1988). In California, sexually dichromatic; incidence of dark coloration higher among females than among males. Among 15 breeding pairs in ne. California, 89% of light-plumaged birds were males but only 33% of dark-plumaged birds were males (Woodbridge 1987). Similarly, in Central Valley, CA, pure light morph (white wing-linings and white body) were males, and only 1 completely dark (dark wing-linings and dark body) bird was a male (Estep 1989). Male of breeding pair tends to be lighter than female (Woodbridge 1987, Estep 1989).

Three age classes can often be distinguished in the field, Defintive plumage being assumed following the Third Prebasic Molt (Cameron 1913, Oberholser 1974, Wheeler and Clark 1995, Pyle 2008); however some individuals may acquire it following the Second Prebasic Molt (Coues 1874, Palmer 1988, Woodbridge et al. 1995a, Pyle 2008). Suggestions that definitive plumage is not attained until the fourth cycle (Cameron 1913, Oberholser 1974) are probably not accurate (Pyle 2008). In ne. California, two of five 3-yr-old breeding birds were in Predefinitive plumage (Woodbridge et al. 1995a). Sexes generally similar in appearance, but females larger than males (Sarasola and Negro 2004), more often dark, and have more distinct bars and wider dark subterminal band in the rectrices (Pyle 2008).

Following based primarily on detailed plumage descriptions in Friedmann (1950), Roberts (1955), Oberholser (1974), Palmer (1988), Wheeler and Clark (1995), and Wheeler (2003a, 2003b); see Pyle (2008) for age-related criteria.

Natal Down

Present Jun-Aug. Hatchlings covered with thick protoptile (first) down at birth, described as white with slight yellowish tinge (Bent 1937); yellowish white or grayish white, tinged with pale pinkish buff on head and middle of back (Oberholser 1974); or pale white (Fitzner 1978). Mesoptile (second) down begins to replace protoptile down 6 d after hatching (Fitzner 1978); described as white above and below (Oberholser 1974) or imparting gray cast (Fitzner 1978). Generally downy in overall appearance until about 18 d after hatching.

Juvenal (First Basic) Plumage

Present Sep-Nov/May. Characterized by uniform wear to all feathers including primaries, secondaries, and rectrices (Pyle 2005b, 2006, 2008).

Light Morph: forehead and throat whitish merging into dark brown streaked crown and somewhat dark streaked buffy superciliary line and buffy cheeks; chin whitish with dark brown median streaking; narrow eye-line and broader malar streak dark brown, both merging with dark brown on side of neck and side of upper breast. Feathers of upper back buffy with dark brown centers (back appears dark streaked); feathers becoming increasingly brown down back until all brown on lower back. Scapulars and wing-coverts dark brown margined with buffy, most broadly on scapulars; buffy margins of scapulars scalloped, include cinnamon buff tones. Brown portion of wing-coverts with broad, indistinct, pale bands. Remiges fuscous, narrowly banded dark brown with bands increasing in width distally; outermost 4 or 5 primaries largely black distally and all remiges tipped with white, except white reduced or absent on outermost 2–4 primaries (among p7-p10). Basal portion of outermost 3 primaries distinctly paler than basal portion of remaining remiges. Underwing-coverts whitish with cinnamon buff tones, variably marked with dark brown; in flight, underwing coverts contrast with darker remiges. Rectrices fuscous, banded dark brown, and with broad subterminal black band and whitish tip; dark bands increase in thickness distally. Underparts creamy white or with some ochraceous tawny to cinnamon buff tones; streaked with dark brown, most heavily along sides of lower throat and upper breast; lower belly largely unmarked except for dark brown barring sometimes along lower flanks and outer undertail coverts. Feather wear causes plumage to lighten on head and scapulars by spring (sometimes considerably) while reducing pale feather margins on remaining upperparts.

Dark Morph: similar but more heavily marked with brown on underparts and underwing coverts and/or with rufous spotting on underparts. Underparts may be densely streaked with hint of brown band, or “bib,” across throat and upper breast. Undertail coverts, however, remain primarily whitish in all darker individuals, contrasting with darker belly.

Formative Plumage

Absent (in 50% of birds) or present Nov/Feb-Jun/Aug. Similar to Juvenal Plumage in both morphs but some body feathers fresher and intermediate in appearance between those of Juvenal and Definitive Basic plumages, the feathers of chest in light-morph often darker. Uniformly juvenal wing and tail feathers retained.

Second Basic Plumage

Present Jun/Aug-Jul/Sep. Similar to Juvenal Plumage in both morphs, but wings and tail (in all morphs) have wider and more prominent dark subterminal band; in light morph, birds are paler and streaked underparts less pronounced, and in dark morph birds are less spotted with white and more uniformly dark (MJB). Some juvenal body (especially rump) feathers and/or wing coverts, 1-5 juvenal outer primaries (among p6-p10), 1-7 juvenal secondaries (among s3-s4 and s6-s10), and/or 1-4 juvenal rectrices (usually among r2 and r5) retained, contrastingly worn, and showing aspect of Juvenal Plumage (above), the secondaries and primaries narrower and paler (more bleached) and rectrices with more distinct barring (illustration in Pyle 2008). Small proportion (~15-20%) of individuals can undergo a complete molt and can be indistinguishable from birds in Definitive Basic Plumage.

Definitive Basic Plumage

Present Jul/Sep-Jul/Sep. Characterized by mixed generations of basic body (especially rump) feathers, and 2-5 sets of basic feathers among primaries and secondaries, in Staffelmauser patterns; replacement sets in primaries defined by a worn feather immediately distal to a fresh proximal feather, and those of secondaries showing mixed generations in various sequences (Clark 2004; Pyle 2005b, 2006, 2008). Number of sets equates to minimum ages of 2-5 years.

Light Morph: Appearance of light morphs quite variable, but most have similar plumage pattern. (1) Head slaty gray with white chin (sometimes extending onto throat) and whitish forehead (sometimes continuing between cere and eye). Sides of neck, lower throat, and upper breast solid rufous, forming distinctive band, or “bib.” Lower breast to tail white toward buffy, with a few rufous streaks. Upperparts, including wing-coverts, uniform muted dark gray brown; feathers (except lesser wing-coverts) have light margins that wear away with time. Uppertail coverts paler, forming U-shaped area at base of tail. Rectrices grayish with white tips, crossed throughout by 9–12 narrow dark bands that increase in thickness distally; last subterminal dark band very broad. In flight, underwing-coverts white, indistinctly marked rufous with blackish “carpal patch” formed by blackish markings on tips of outermost underprimary-coverts. Contrast between light underwing-coverts and dark remiges are distinctive.

Other variations occur: (2) Breast band same gray color as head; entire belly barred rufous; undertail coverts white but with dark bars on outermost feather; axillars same as belly; and underwing coverts marked throughout with dark barring. (3) Head and breast-band brown; breast band partially barred white (or band reduced to dark spots) and throat narrowly streaked dark brown; belly sparsely streaked dark brown; underwing coverts marked with brown and rufous.

Intermediate (Rufous) Morph. (1) Similar to light morph, except breast band dark gray or brown; lower breast and belly solid rufous, variously streaked or barred with gray. (2) Lower throat, breast, and belly solid rufous with some dark gray streaking. Throat narrowly streaked dark gray or brown; underwing coverts whitish to rufous. Undertail-coverts whitish with dark bars on outermost feathers.

Dark Morph. Body feathering entirely blackish brown; white markings on face reduced or absent but with some chestnut markings (lower belly, superciliary line); lower sides may be barred. Remiges similar to those of light morph, but darker overall; underwing-coverts vary from white with variable amount of rufous mottling, to blackish with variable amount of whitish mottling. Rectrices similar to those of light morph, but dark barring heavier throughout. Undertail coverts white with blackish barring, varying from light to heavy primarily on outermost feathers.

Bare Parts

Bill, Cere, And Gape

Bill slaty to blackish, becoming dull blue or pale olive buff at corner of mouth. Cere and mouth-lining pale greenish yellow to yellow.

Iris

Gray or blue gray in Juvenal plumage; becoming yellowish in first year, pale brown in second year, and dark brownish to reddish-brown in adults (Pyle 2008).

Legs And Feet

Vary from wax yellow (Brown and Amadon 1968) to creamy to light grayish green (Johnsgard 1990). Leg color of immature birds generally paler than adults (Clark and Wheeler 1987).

The most critical research questions continue to focus on the ecology of Swainson’s Hawks during migration and in South America. Basic information is needed to learn if the conversion of tropical rain forests and southern grasslands to pasture and farmland has affected their distribution. Based on new information about their distribution in the nonbreeding grounds, the demographics of Swainson’s Hawks in South America should be studied to determine sources of mortality, especially the extent of pesticide use outside of Argentina. Habitat requirements and use patterns south of the breeding grounds should be coupled with an analysis of habitat distribution, existing land use practices, and likely short- and long-term changes in land use. Foraging habits and metabolism prior to and during migration also require additional study, especially the reliance on outbreaks of grasshoppers and large flocks of dragonflies. The temporal and geographic distributions of these food sources remain relatively unknown.

On the breeding grounds, research is needed to learn why populations and productivity have declined, especially in areas where apparently suitable habitat remains unoccupied. Studies of apparent declines in populations of ground squirrel and other prey species should be undertaken, and effective monitoring of Swainson’s Hawk populations is needed to follow trends. Long-term demographic studies such as those in ne. California (B. Woodbridge pers. comm.) and the Central Valley of California (J. A. Estep pers. comm.) should be continued and duplicated in other parts of this hawk’s breeding range. The ecology of fledglings immediately prior to the start of migration is virtually unknown and warrants study. Sampling of prey species numbers must be initiated, together with continued periodic sampling of contaminant levels. The effects of climate change on migratory and breeding phenology should be examined.

Locomotion

Walking, Hopping, Climbing, Etc

Adults and fledglings regularly observed on ground in pursuit of insects. Run quickly and smoothly, with head slightly lowered and often with wings slightly extended and raised. May also attempt to flush prey by jumping and leaping on ground while simultaneously flapping wings. May catch prey on ground using either beak or talons (Ridgway 1887, Fitzner 1978, Palmer 1988, Sarasola and Negro 2005).

Flight

Strong, buoyant, and graceful direct flapping flight with moderately deep wing-beats when not transporting prey. Soaring flight with wingtips elevated above back in shallow dihedral similar to that of Turkey Vulture and Zone-tailed Hawk (Buteo albonotatus). Will hover or “kite” (hang motionless in air) when foraging, especially in moderate to strong winds (Clark and Wheeler 1987, Palmer 1988), but hovering bouts shorter and less frequent than those of White-tailed Kite (Elanus leucurus) or Rough-legged Hawk (MJB). s direct flight, soaring, and hovering when foraging. In migration, relies heavily on thermal updrafts (see Migration, above). Migratory birds begin moving when air temperature is sufficient to create thermals. In rising air, spirals upward with wings fully spread, outer primaries separated, and tail at least half open. Where air cools at top of thermal, folds primaries toward rear, closes tail, and glides downward at low angle, exchanging altitude for distance until next thermal is located (Palmer 1988).

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, Anting, Etc

Preens while perched. Last week before fledging, nestlings spend more time preening than at any other behavior, including sleep (Porton 1977). In Idaho, flock of up to 238 nonbreeding birds bathed in reservoir on warm Aug afternoons, then perched and preened until sunset (Johnson et al. 1987). No other quantitative information available. Allopreening (including between nestlings) not reported (Porton 1977, Fitzner 1978). No information on stretching or anting.

Sleeping, Roosting, Sunbathing

Communal nocturnal roosts of ≥100 immatures and nonbreeding adults reported; during breeding season, roost in strands of riparian vegetation and large groups of native oaks in central California (J. A. Estep pers. comm.), and in cottonwoods (Populus spp.) in Idaho (Johnson et al. 1987). Similar-sized roosts of premigratory flocks also reported from Central Valley, CA (J. A. Estep pers. comm.), and from sw. Oklahoma, where shelterbelts are used. Approximately 30 birds wintering in Sacramento–San Joaquin River delta of California in 1993–1994 used a single windbreak of exotic eucalyptus trees as nocturnal roost (Herzog 1996). Nocturnal roosts in Argentina averaged 2,300 individuals/roost (range 25–7,000, n = 6) (Woodbridge et al. 1995b) and 658 individuals/roost (range 8–5,000, n = 27) (Sarasola and Negro 2006). Roosts are located in windbreaks and groves (0.5–30 ha) of exotic tree species, mainly eucalyptus but also elm, pine and cypress. (Sarasola and Negro 2006). A single roost at La Chanilao near General Pico, La Pampa Province, was estimated to contain 12,000 individuals (M. Goldstein and B. Woodbridge pers. comm.). White et al. (1989) also reported use of eucalyptus in Argentina. Small flocks in semiarid areas of central Argentina indicate use of native forest for foraging and roosting (JHS). No information on roosting behavior of breeding adults away from nest or on microhabitat of roost sites. No information on sunbathing.

Daily Time Budget

Apparently strictly diurnal, even during migration. Needs study. No quantitative time budget information reported, but like most other North American buteos, appears to spend majority of time either perched or at nest. Time budget in wintering grounds examined only for foraging behavior (Sarasola and Negro 2005). In Argentina hawks spend more time hunting in the air during mid-day when air temperatures are high and thermals frequent. Ground foraging only occurs early in the morning and late in the afternoon when air temperatures decrease.

Agonistic Behavior

All reported information is associated with apparent intra- or interspecific territorial defense; see Spacing, below.

Spacing

Nests spaced typically at least 1.5–2.5 km apart (Table 2). Shortest average internest distance (1.14 km) reported from Central Valley, CA, where foraging habitat is fairly uniformly distributed, but nest sites are limited to narrow riparian bands and isolated trees within extensive agriculture (Estep 1989). During a 10-yr monitoring study in Central Valley, CA, 5 nests typically found along a particular riparian strip only 1 km long, with nearest nests only 60 m apart (J. A. Estep pers. comm.). Similarly, in ne. California, nests within isolated patches of junipers surrounded by alfalfa fields ranged from 0.4 to 1.1 km apart (Woodbridge 1987). See Demography and populations: population status, below, for data on nesting density.

Territoriality

No information on size or characteristics of defended territories, but in central California, aggressively defends only small area around nest from conspecifics and other buteos (J. A. Estep pers. comm.). Returns to breeding grounds in North America when most other birds of prey are already nesting; strong interspecific territorial interactions reported. In n.-central Oregon, Red-tailed Hawks are incubating eggs when Swainson’s Hawks arrive in early–mid-Apr and establish territories within 24 h; interspecific aerial combat lasts almost entire first day after arrival, decreases sharply the second day, but defense continues throughout breeding season (Janes 1984). In central California, returning Swainson’s Hawks may successfully dislodge incubating Red-tailed Hawks or White-tailed Kites that are using Swainson’s Hawk nests from previous years; may be responsible for substantial nesting failure of White-tailed Kites; attempts to displace incubating Great Horned Owls (Bubo virginianus) are usually unsuccessful (J. A. Estep pers. comm.). Near Calgary, Alberta, typical territorial defense against other Swainson’s Hawks or Red-tailed Hawks consists of the following behaviors: A male, usually calling loudly, flies toward approaching hawk; territorial bird with lowered talons either strikes or swoops at approaching hawk ≥1 times; and both birds circle to gain altitude until approaching hawk reaches vertical limit of territory (Rothfels and Lein 1983). Of 31 territorial interactions involving Swainson’s Hawks, 5 were directed at other Swainson’s Hawks and 26 at Red-tailed Hawks; females rarely observed in territorial defense (Rothfels and Lein 1983). In Idaho, where nesting near Ferruginous Hawks, “cooperative” territorial defense suggested by both species defending the nest of the other against intruding humans, including 1 instance of a Swainson’s Hawk swooping down on an investigator who was climbing to a Ferruginous Hawk nest and 1 instance of both species simultaneously directing territorial defense at intruding Golden Eagle (Aquila chrysaetos; Thurow and White 1983).

Other intruders that may be harassed and driven away from active nest include Turkey Vulture, Northern Harrier (Circus cyaneus), Golden Eagle, American Kestrel (Falco sparverius), Great Horned Owl, Long-eared Owl (Asio otus), Chihuahuan Raven (Corvus cryptoleucus), and American Crow (Corvus brachyrhynchos; Porton 1977, Fitzner 1978, Palmer 1988, (CSH). On 2 occasions, attacked and knocked fledgling Long-eared Owls to ground, and in 1 instance struck Northern Harrier on back; all of the attacked birds flew off apparently uninjured (Fitzner 1978). Strength of defense decreases throughout breeding season; may be extremely tolerant of other species, as evidenced by pair of Common Ravens (Corvus corax) that successfully fledged young 3 consecutive years from nest within 75 m of successful Swainson’s Hawk nest (Fitzner 1978). Nest defense by adults may continue for weeks after loss of eggs or young (Woodbridge 1987).

Individual Distance

No quantitative information available, but found in close association with other individuals throughout year, except during nesting season near nest site; even then, flies and forages with others away from immediate vicinity of nest.

Sexual Behavior

Mating System And Sex Ratio

Usually monogamous. Probable instances of polygyny observed near Calgary, Alberta, with 3 adults present at single nests throughout breeding seasons in 1987 and 1988 (Cash 1989), and near Kerrobert, Saskatchewan, where a distinctive erythristic male defended 2 females at active nests about 200 m apart (CSH). Low incidence of polyandry observed between 1984 and 1995 in ne. California (see Breeding: cooperative breeding, below).

No information on sex ratios at hatching or fledging, or within regional populations.

Pair Bond

Courtship Displays. Described as vigorous and acrobatic (Olendorff 1974a, Porton 1977, Fitzner 1980). Begins with male and female soaring in circles 400–500 m in diameter 80–150 m above nest site, with wings and tail spread and little flapping. Smaller bird, presumably male, then closes its wings, breaks from soar, and begins rapid dive. Descent ends with open-wing, upward recovery that may be followed by regaining altitude and resuming soaring, rapid flying in tight circle, or series of dives and recoveries eventually ending with male perched at nest, in nearby tree, or on ground. Female may or may not join male at perch, and copulation may or may not ensue. Pattern of this “sky-dance” display is not strongly stereotyped, but the tight circling, displaying of underwing and light-colored feathers at base of tail, and steep dives are typical.

Copulatory Displays. Copulatory behavior described by Fitzner (1978). Female, usually on dead horizontal limb, sat quietly as male flew from another perch. Female assumed Solicitation Posture, squatting on limb with neck outstretched, head tilted downward, wings lowered, and tail raised near vertical. Male landed directly on her back, resting on tarsi with toes in a fist. Male held wings outstretched, fluttering lightly to keep balance. Male lowered partially fanned tail beneath tail of female and bent head down toward neck of female. Copulation averaged 7.2 s (range 4–22, n > 50). After copulation, male dismounted and remained with female for approximately 1 h; copulated whenever solicited by female. Between copulations, usually would meticulously preen feathers. Postcopulatory activities included nest-building, hunting, soaring, and harassment of other species of raptors. During copulation, one bird (presumably a female) would call (see Sounds: vocalizations, above).

Duration of Pair Bond. Pairs color-banded in 1986 and 1987 in central California mated with same mate for several years (mean 5.0 yr ± 0.9 SE ), suggesting long-term monogamy between birds; 2 pairs were still returning after 9 and 10 yr when these data were summarized (Estep 1989, pers. comm.). Interchanging territories and mates between seasons has been observed in ne. California, however (Woodbridge in Estep 1989). In 3 instances, females remated within 1–3 d following experimental removal of mate (Schmutz 1977).

J. K. Schmutz (in Palmer 1988) reports that copulations occur repeatedly and in variety of locations, including trees and fence posts. Coplations occur in morning (most frequent), at midday (least frequent), and in evening (Porton 1977, Fitzner 1978). Most copulations during nest-building period, but also rarely during incubation (Porton 1977, Fitzner 1978).

Extra-Pair Copulations

In ne. California, observed occasionally among adjacent territory members as well as with second male in polyandrous groups (B. Woodbridge pers. comm.).

Social And Interspecific Behavior

Degree Of Sociality

Most gregarious of North American raptors. Premigratory foraging flocks of >100 birds in breeding areas are common. Migratory flocks, foraging aggregations, and nocturnal roosts in South America may include thousands of individuals (see Migration). During breeding season in central California, nonbreeding birds may form flocks of >100 birds that forage together and use communal nocturnal roosts (J. A. Estep pers. comm.). Breeding pairs are usually monogamous and solitary, but frequently forage with other individuals near or away from active nests, usually in response to farming activities.

Play

Two adults, presumably a breeding pair, will repeatedly drop and catch prey item in air (A. S. England, pers. comm.).

Nonpredatory Interspecific Interactions

Frequently nests in same tree with wide variety of other birds, especially Mourning Doves (Zenaida macroura), Western Kingbirds (Tyrannus verticalis), and Yellow-billed Magpies (Pica nuttalli). House Sparrows (Passer domesticus) sometimes nest on underside of active Swainson’s Hawk nest (CSH). A pair of Long-eared Owls successfully fledged 5 young from nest 6 m below Swainson’s Hawk nest that fledged young (Fitzner 1978). Often harassed by other species nesting near active nests, especially American Kestrel, Western Kingbird, Eastern Kingbird (Tyrannus tyrannus), American Crow, and Northern Mockingbird (Mimus polygottos; Cameron 1913, Porton 1977, Fitzner 1978,). In 1 instance, “attacking” Eastern Kingbird landed on back of Swainson’s Hawk and was carried for several seconds (Cameron 1913). See also Spacing, above, for description of interspecific territoriality with Red-tailed Hawks.

Migrates through Central America in mixed flocks with Turkey Vultures and Broad-winged Hawks (Smith 1980), but no reported evidence of either cooperative or antagonistic interactions in these aggregations. One reported incident of kleptoparasitism on American Kestrel with a small mammal (Woffinden 1986). See also Agonistic behavior, above.

Predation

No information on predation of adults. In Wyoming, 3 of 49 clutches lost to American Crows (Dunkle 1977). In se. Washington, 2 fledglings lost to Great Horned Owls and 1 to either coyote (Canis latrans) or bobcat (Felis rufus; Fitzner 1978). In ne. California, 5 nests failed because of nestling predation by Great Horned Owls, and 2 because of predation by Golden Eagles (n = 208; Woodbridge 1991).