Buteo swainsoni

Swainson’s Hawk (Buteo swainsoni) is a medium-sized slender hawk (Crow-sized), with long, pointed wings and a long tail. Measurements include: length 17-22 inches (43-56 cm); wingspan 47-54 inches (120-137 cm); weight 1.3-2.7 lb (595-1240 g). Females slightly larger than males. Plumage extremely variable, but most individuals are recognizable. Adult-sides of the head and entire upper parts dark blackish brown; feathers obscurely edged with paler brown to cinnamon. Tail gray, basally whitish, with a narrow white tip, and several indistinct blackish bars, the last one broader. Primaries blacker than back; becoming paler basally. Throat white; breast brownish chestnut with weak black shaft streaks. Belly and legs dull white; indistinctly mottled and barred with brown to rufous. Under-wings pale with conspicuous dark marks at ends of coverts. Dark phase more or less sooty all over. Wing and tail as in normal phase, except that wing linings are much more marked with blackish. Rufous phase lighter brown below than the dark phase; and somewhat barred and blotched below with rusty brown. Intermediates occur between all the phases. Eye dark brown; cere pale greenish yellow; bill blackish; legs wax yellow (Brown et al 1968).

The immature plumage, which is worn for two years, is similar to that of adults in its two- toned underwing and finely barred tail, but young birds have a spotted and streaked breast that at times shows a hint of a darker pattern, and the head shows a definite buffy streak above the eye and on the cheek, with a dark eye line and malar stripes. This typical pattern occurs on perhaps half the Swainson’s Hawk encountered in Arizona, and if color pattern alone is used for identification, the other half will be mis-identified. (Glinski 1998).

Found only in the New World; it breeds in North America, in the Great Plains and arid regions, north sparingly to interior Alaska, and south to northern Mexico, and winters in South America. The normal winter range is the Pampas of Argentina, and it has been assumed that any found elsewhere at that season are casuals, probably unable to make the long migration (Brown et al 1968).

Gives a descending shrill, plaintive whistle, kreeeeeeer, trailing off at end. In flight, shows profile like that of Turkey Vulture; the wings are held in a dihedral, or "V", position, which promotes aerodynamic stability in open landscapes where wind can interfere with flight close to the ground. Highly migratory, often seen in large flocks on spring and fall flights. During the breeding season, a soaring, open country hunter. Sometimes hunts high in the air, but more frequently courses low over prairie. Rarely observed flying low at high speed as Ferruginous Hawk does. Often hunts from perches such as tree limbs, poles or posts, rocks, and elevated ground.

W N America; winters to n Argentina, s Brazil and Paraguay.

The breeding range of the Swainson's hawk is restricted primarily to
western North America from interior Alaska and western Canada south
into northern Mexico [12,21].  The Swainson's hawk winters primarily on
the pampas of southern South America, irregularly north to Costa Rica
and Panama, and sometimes north to the southwestern United States and
southern Florida [1,12,21].  During migration the Swainson's hawk occurs
regularly in most of the central states and Canadian provinces, and
rarely, east along the Gulf Coast to Florida. It is occasionally a fall
migrant through the Florida Keys.  The Swainson's hawk is occasionally
found in northeastern North America from southern Ontario, southern
Quebec, New York, and Massachusetts south to Virginia [1].

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This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

    1  Northern Pacific Border
    2  Cascade Mountains
    3  Southern Pacific Border
    4  Sierra Mountains
    5  Columbia Plateau
    6  Upper Basin and Range
    7  Lower Basin and Range
    8  Northern Rocky Mountains
    9  Middle Rocky Mountains
   10  Wyoming Basin
   11  Southern Rocky Mountains
   12  Colorado Plateau
   13  Rocky Mountain Piedmont
   14  Great Plains
   15  Black Hills Uplift
   16  Upper Missouri Basin and Broken Lands

AK AZ
CA CO CT DE FL ID IL IA KS LA MD MA MT NE NV NH NJ NM NY ND OH OK OR PA RI SD TN TX UT VA WA WY
AB BC MB NT ON PQ SK YT
MEXICO

The Swainson's Hawk, -Buteo swainsoni-, spends most of the year in the western United States extending into southwest Canada and south to west Texas. In the winter months, these birds migrate over Central America to the La Pampas region of Argentina (Brown 1996, TPWD 1997).

Biogeographic Regions: nearctic (Native ); neotropical (Native )

occurs (regularly, as a native taxon) in multiple nations

Canada

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Breeding

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Breeding

Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDS: known to have bred in east-central Alaska east into Yukon Territory and extreme northwestern Mackenzie; central Alberta, central Saskatchewan, southern Manitoba, western and southern Minnesota, and western Illinois south (mainly east of Cascades and Sierra Nevada) to southern California (rarely), Baja California (formerly), Sonora, Durango, Chihuahua, central and southern Texas and western Missouri; eastern breeding limits unstable. WINTERS: according to AOU (1983), primarily on pampas of southern South America (south to Uruguay and Argentina), irregularly north to Costa Rica and Panama, casually or irregularly north to the southwestern U.S. (especially Texas) and southern Florida.

This hawk's most unique feature is its variation in color. The light color morph includes white patches on the forehead, the throat and the belly. The rest of the body is a dark brown. The dark color morph, which is the less common type, includes an entirely dark brown body with only a white patch under the tail. Other variations between these two distinct extremes have been observed. These hawks vary in length from 19 to 22 inches, and have a wingspan of 47 to 57 inches. An average weight for a male is 1.8 pounds, while the average for the female is almost 2.5 pounds. This bird is commonly confused with a Red-tailed hawk, but the Swainson's Hawk has a longer wingspan, more variation in color, and flies in a slight dihedral pattern (Brown 1996, AID 1997).

Average mass: 980.64 g.

Length: 53 cm

Weight: 1069 grams

Comments: Savanna, open pine-oak woodland and cultivated lands (e.g., alfalfa and other hay crops, and certain grain and row croplands) with scattered trees. Tolerates extensive cultivation in nesting area (Schmutz 1989), though vineyards, orchards, rice, corn, and cotton are not suitable foraging habitat. In migration and winter also in grasslands and other open country (AOU 1983). Migrants may roost at night on ground in very large fields (Ridgely and Gwynne 1989). Nests typically in solitary tree, bush, or small grove; many nests on old black-billed magpie nests; sometimes on rock ledge. Readily nests in trees in shelterbelts and similar situations produced by humans (Gilmer and Stewart 1984). Recently reported nesting in city trees and on railway signal gantry in Regina, Saskatchewan (Condor 94:773-774). In the Central Valley of California, nests often are within one mile of a riparian zone; Great Basin nests, usually in junipers, are not near riparian zones (Biosystems Analysis, Inc. 1989). Evidently often returns to area where it nested in previous year.

GREAT BASIN AND MOJAVE HABITAT:
Swainson's Hawks have adapted to agricultural landscapes in Nevada. An ideal landscape for the Swainson's Hawk provides large riparian nesting trees, agricultural fields, and open shrublands within relatively close proximity (GBBO 2010). Swainson's Hawks in the Great Basin occupy the Juniper/Sagebrush community typical to the area. In California, Swainson's hawk habitat generally consists of large, flat, open, undeveloped landscapes that include suitable grassland or agricultural foraging habitat and sparsely distributed trees for nesting (England et al. 1997). Populations in the Great Basin often use juniper trees (Juniperus sp.) for nesting (England et al. 1997), and at least three known nest sites in the Mojave Desert are in Joshua trees (Yucca brevifolia) (California Natural Diversity Database 2009) (PCCP 2010). In addition to Joshua trees, this species was also known historically from the Mojave Yukka (Yucca schidigera) and possibly desert riparian habitats (Bloom 1980).

More info for the terms: cover, density, selection

Swainson's hawk nests are often built in trees that provide shade for
the nest but also afford a good view of the surrounding terrain [33].

The Swainson's hawk is a more efficient predator in open areas than in
areas with high vegetative cover [5].  Bechard [5] found that vegetative
cover is more important than prey abundance in the selection of hunting
sites by the Swainson's hawk.  In Whitman County, Washington, the
Swainson's hawk foraged at sites where vegetative height and density had
been reduced, even though other areas had higher prey density [5].
Alfalfa field use by Swainson's hawk in northern California increased
dramatically during monthly harvests that reduced vegetative heights
[38].

More info for the terms: shrubs, tree, tundra

The Swainson's hawk inhabits mostly semiopen to open areas in tundra,
valleys, plains, dry meadows, foothills, and level uplands at low to
middle elevations [1,31,40].

Nesting habitat - The Swainson's hawk nests almost exclusively in trees
[37] and will nest in almost any tree species of suitable size (taller
than 10 feet [3 m] with a d.b.h. of 2 inches [5 cm] or more) [6,7].
Nests are constructed in isolated trees (dead or live), in trees in
wetlands and along drainages, or in windbreaks in fields and around
farmsteads [6,12,31].  The Swainson's hawk builds nests from 4 to 100
feet (1.2-30.4 m) above the ground [12,14,35].  They sometimes add to an
existing black-billed magpie (Pica pica) nest [31].  The Swainson's hawk
occasionally nests in shrubs, on the crossbars of telephone poles, or on
the ground, low cliffs, rocky pinnacles, or cutbanks [6,12,31].

In the Central Valley of California, the majority of Swainson's hawk
nests and territories are located in or near riparian systems.  Nests
are found most often in cottonwoods and oaks [33].  In Whitman County,
Washington, Swainson's hawk nests were constructed in black locust
(Robinia pseudoacacia), cherry (Prunus spp.) and hawthorn (Crataegus
spp.) trees [5].  Of 48 Swainson's hawk nests on the Laramie Plains,
Wyoming, 43 were in narrowleaf cottonwood (Populus angustifolia),
peachleaf willow (Salix amygdaloides), or other willows.  None of the
few buttes in the area were used for nesting [15].  In the Centennial
Valley of Montana, Swainson's hawks nest extensively in willows [32].
At 234 Swainson's hawk nest sites in North Dakota, eastern cottonwood
(Populus deltoides) was the most common tree species used (45%).  Other
species included Chinese elm (Ulmus parvifolia), 22 percent; peachleaf
willow, 13 percent; boxelder (Acer negundo), 12 percent; and green ash
(Fraxinus pennsylvanica), 7 percent.  American elm (U. americana) and
Russian olive (Elaeagnus angustifolia) accounted for the remaining 1
percent [20].  In the Lower Sonoran Desert of New Mexico, the Swainson's
hawk often nests and roosts on large yucca plants [30].  In the
southwestern United States, mesquite is commonly used [6].

Foraging habitat - The Swainson's hawk generally forages in open
habitats with short vegetation containing small mammals, reptiles,
birds, and insects [6,40,38].  During the nesting season the Swainson's
hawk usually forages within 1.9 miles (3 km) of the nest.  The
Swainson's hawk has a home range of approximately 3.5 square miles (9 sq
km) [40].  Although the Swainson's hawk does search for prey from
elevated perches, it relies much more on aerial foraging.  Consequently,
it is not tied to habitats containing an abundance of perches, and often
occupy habitats with few or no perches except the nest tree [23].

Winter habitat - The Swainson's hawk generally spends the winter south
of the United States [1,12,31]; no information is available in the
English literature on its habitat in Central and South America.

More info for the term: shrubs

The Swainson's hawk breeds in open grasslands, sagebrush (Artemisia
spp.), shrub-steppe, oak (Quercus spp.) woodlands, open pine (Pinus
spp.)-oak woodlands, pinyon-juniper (Pinus spp.-Juniperus spp.)
woodlands, and cultivated lands [1,3,6,16,40].  In California the
Swainson's hawk favors open blue oak (Quercus douglasii) savannahs and
gray pine (Pinus sabiniana)-oak woodlands [35].  In the Central Valley
of California, populations of Swainson's hawks frequently nest and roost
in riparian communities dominated by valley oak (Quercus lobata),
cottonwoods (Populus spp.), California sycamore (Platanus racemosa), and
willows (Salix spp.)  [22,33].  Foraging habitat for Swainson's hawks in
California includes native grassland communities of oat (Avena spp.),
brome grass (Bromus spp.), ryegrass (Elymus spp. and Lolium spp.), and
barley (Critesion spp.) [33]. West of Laramie, Wyoming, Dunkle [15]
reported that breeding habitat of the Swainson's hawk included irrigated
sedge meadows, shortgrass plains with some sagebrush, and black
greasewood (Sarcobatus vermiculatus) [15].

In the Great Basin, the Swainson's hawk is often found nesting in
juniper-sagebrush and prairie habitats [33].  In Arizona, the Swainson's
hawk generally occurs in sparse semidesert grasslands, plains
grasslands, Great Basin grasslands, and Chihuahuan Desert scrub often
mixed with a few species of shrubs including yucca (Yucca spp.),
creosotebush (Larrea tridentata), mesquite (Prosopis spp.), and fourwing
saltbrush (Atriplex canescens).  In New Mexico and Texas, breeding
Swainson's hawk occur in various types of grasslands including
grasslands with sand shinnery oak (Q. havardii), and are occasionally
found in Chihuahuan Desert scrub.  In Oklahoma, Swainson's hawk breed
primarily in grasslands [6].

The Swainson's hawk sometimes nests in intensively cultivated areas
[4,5,20].  Of the large raptors breeding in northern Colorado, only the
Swainson's hawk regularly nested near cultivated lands [20].

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This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

More info for the terms: cactus, shrub

   K023  Juniper - pinyon woodland
   K024  Juniper steppe woodland
   K026  Oregon oakwoods
   K027  Mesquite bosque
   K030  California oakwoods
   K031  Oak - juniper woodlands
   K032  Transition between K031 and K037
   K033  Chaparral
   K034  Montane chaparral
   K035  Coastal sagebrush
   K037  Mountain-mahogany - oak scrub
   K038  Great Basin sagebrush
   K039  Blackbrush
   K040  Saltbush - greasewood
   K041  Creosotebush
   K042  Creosotebush - bursage
   K043  Paloverde - cactus shrub
   K044  Creosotebush - tarbush
   K045  Ceniza shrub
   K047  Fescue - oatgrass
   K048  California steppe
   K050  Fescue - wheatgrass
   K051  Wheatgrass - bluegrass
   K053  Grama - galleta steppe
   K054  Grama - tobosa prairie
   K055  Sagebrush steppe
   K056  Wheatgrass - needlegrass shrubsteppe
   K057  Galleta - three-awn shrubsteppe
   K058  Grama - tobosa shrubsteppe
   K059  Trans-Pecos shrub savanna
   K060  Mesquite savanna
   K061  Mesquite - acacia savanna
   K062  Mesquite - live oak savanna
   K063  Foothills prairie
   K064  Grama - needlegrass - wheatgrass
   K065  Grama - buffalograss
   K066  Wheatgrass - needlegrass
   K067  Wheatgrass - bluestem - needlegrass
   K068  Wheatgrass - grama - buffalograss
   K069  Bluestem - grama prairie
   K070  Sandsage - bluestem prairie
   K071  Shinnery
   K072  Sea oats prairie
   K074  Bluestem prairie
   K075  Nebraska Sandhills prairie
   K076  Blackland prairie
   K077  Bluestem - sacahuista prairie
   K083  Cedar glades
   K085  Mesquite - buffalograss
   K086  Juniper - oak savanna
   K087  Mesquite - oak savanna
   K088  Fayette prairie

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This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES37 Mountain meadows
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands
FRES41 Wet grasslands
FRES42 Annual grasslands

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This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

    66  Ashe juniper - redberry (Pinchot) juniper
    67  Mohrs (shin) oak
    68  Mesquite
   203  Balsam poplar
   217  Aspen
   220  Rocky Mountain juniper
   221  Red alder
   222  Black cottonwood - willow
   233  Oregon white oak
   235  Cottonwood - willow
   236  Bur oak
   238  Western juniper
   239  Pinyon - juniper
   240  Arizona cypress
   241  Western live oak
   242  Mesquite
   246  California black oak
   249  Canyon live oak
   250  Blue oak - Digger pine
   252  Paper birch
   255  California coast live oak

Systems

• Terrestrial

This hawk prefers open grasslands and desert-like habitats. It is common to see this hawk perched on a fence post in a prairie or open range. The Swainson's Hawk also inhabits agricultural areas, and is known to follow farmer's tractors in search of insect or rodent prey (Brown 1996, AID 1997).

Terrestrial Biomes: desert or dune ; savanna or grassland ; chaparral

Non-Migrant: No. All populations of this species make significant seasonal migrations.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: Yes. At least some populations of this species make annual migrations of over 200 km.

In migration, occurs regularly in most of Middle America, and rarely east along the Gulf Coast to Florida (AOU 1983). In California, migrates March-early May, with a peak in the first half of April, and September-October (Biosystems Analysis, Inc. 1989). Migrants are greatly concentrated as they pass through Panama (mostly March-early April and October-early November; Ridgely and Gwynne 1989). Migrates through Costa Rica late September-November and late February-early May (Stiles and Skutch 1989). In Colombia, flocks of various sizes reported mainly February-March and September-early November (Hilty and Brown 1986). Main northward migration passes through Panama in mid-March, Veracruz in latter half of March and early April, southern Texas and southwstern U.S. chiefly in April (Palmer 1988); fall concentrations and movements occur in August-September in the north, mainly early October in Texas; peak in migration occurs in September in the southwestern U.S.; arrives in Argentina in late November (Palmer 1988). Annual migration flight may be 18,000-27,000 km, encompasses 4 months of the year. See Houston (1990) for information on migrations of Saskatchewan breeders. Migrates in large, often immense, flocks. Migrates over terrain where updrafts provide needed buoyancy for soaring. May roost at night on ground during migration.

The Swainson's hawk is a versatile and opportunistic predator on
relatively small prey [6].  The Swainson's hawk feeds on small mammals,
large insects, birds, and reptiles [9,14,31,35].  During the breeding
season, the Swainson's hawk primarily preys on small mammals, especially
young ground squirrels (Spermophilus spp.), pocket gophers (Thomomys
spp.), and some microtines [15,20,31,32].  During migration
invertebrates often make up over 90 percent of the Swainson's hawk's
diet [31].

In a North Dakota study, Swainson's hawks preyed primarily on northern
pocket gopher (Thomomys talpoides), Richardson's ground squirrel
(Spermophilus richardsonii), meadow vole (Microtus pennsylvanicus), and
thirteen-lined ground squirrel (S. tridecemlineatus) [20].  To a lesser
extent Swainson's hawks also ate western meadow lark (Sturnella
neglecta), chestnut-collared longspur (Calcarius ornatus), sharp-tailed
grouse (Tympanuchus phasianellus), short-eared owl (Asio flammeus),
American kestrel (Falco sparverius), blue jay (Cyanocitta cristata),
and rock dove (Columba livia) [20].  Toads (Bufo spp.) and various
lizards, mostly desert grassland whiptail (Cnemidophorous uniparens)
and spiny lizards (Sceloporus spp.), were commonly taken by nesting
Swainson's hawk in Arizona.  Mammals, particularly cottontails
(Sylvilagus spp.), ground squirrels, and kangaroo rats (Dipodomys spp.),
were the most common food items brought to Swainson's hawk nestlings in
New Mexico [6].  Rabbits comprised between 40 and 80 percent of the diet
of Swainson's hawk nestlings in New Mexico [7].

The Swainson's Hawk is somewhat of a generalist, and eats whatever it can find. During its time in North America, its diet consists of insects, small mammals and birds, and occasional reptiles and amphibians. When these birds migrate to the Argentina area, they feed mainly on insects like grasshoppers and crickets (Brown 1996, TPWD 1997).

Comments: Vertebrates (mainly mammals) dominate the diet during the breeding season; invertebrates (especially crickets and grasshoppers) are common food at other times and sometimes for nonbreeders in summer. Hawks wintering in Argentina ate mainly dragonflies (Condor 95:475-479, Wilson Bull. 105:365-366). Mammals consumed often include young ground squirrels and pocket gophers. Depending on availability, also eats other small mammals, snakes, lizards, birds, amphibians, and some carrion (e.g., road kills). Hunts for insects on ground; may also catch insects in air. Hunts while soaring or from perch. Does not feed during most of migration (occasional feeding during initial and terminal stages) (Palmer 1988).

Large raptors including great horned owl (Buteo virginianus) and golden
eagle (Aquila chrysaetos), and coyote (Canis latrans), bobcat (Lynx
rufus), and striped skunk (Mephitis mephitis) have been known to kill
Swainson's hawk nestlings and fledglings or destroy clutches
[7,11,15,31,38].  Crows (Corvus spp.) sometimes destroy clutches
[15,31].

Buteo swainsoni preys on:
Lepus californicus
Lepus townsendii
Arvicolinae
Geomyidae
Dipodomys
Sylvilagus
Phasianus colchicus
Spermophilus
Eremophila alpestris
Aves
Mammalia
Sturnella neglecta
Charadrius vociferus
Charadrius montanus
Calamospiza melanocorys
Peromyscus maniculatus
Serpentes
Athene cunicularia
Nucifraga columbiana
Spermophilus washingtoni

Based on studies in:
USA: California, Cabrillo Point (Grassland)

This list may not be complete but is based on published studies.

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: 21 to >300

Comments: No exact figures.

10,000 to >1,000,000 individuals

Comments: Guesstimated number of breeding pairs in Canada in the early 1990s was 20,000-50,000 (Kirk et al. 1995). Total population may be 350,000-400,000 individuals.

More info for the term: fire suppression

The Swainson's hawk occurs in the following four major fire-dependent
plant associations in the western United States:  grassland, semidesert
grass-shrub, sagebrush grass, and pinyon-juniper [26].

Although fire may reduce potential nest trees, it may enhance the
foraging habitat of Swainson's hawks.  Fires that reduce vegetation
height and create open areas probably increase hunting efficiency by
Swainson's hawks.  Open-habitat raptors such as the Swainson's hawk use
scattered patches of woody vegetation near open foraging areas for
nesting and perching.  However, where extensive invasion of woody
species has occurred, Swainson's hawk foraging habitat may be reduced.
The Swainson's hawk is favored by fires that reduce pinyon-juniper
woodlands [26].  Raptors associated with pinyon-juniper woodlands depend
upon edges of openings created by fire and scattered islands of unburned
woodlands [13].  Fire suppression in pinyon-juniper habitats of the
Great Basin of California may have reduced suitable Swainson's hawk
habitat in this area [26].

Regular burning helps to maintain habitat for many prey species of
Swainson's hawk [13,25].  Several studies indicate that many prey
populations increase rapidly subsequent to burning in response to
increased food availability [13].  Fire suppression in grasslands was
detrimental to small bird and mammal populations due to organic matter
accumulation and reduced plant vigor [36].  The Swainson's hawk has been
observed hunting on recently burned areas in Colorado county, Texas [2].
On the Bridger Teton National Forest, Swainson's hawks were more
commonly observed using a high-severity fall burn than a low-severity
spring burn in the same area [27].

More info for the term: tree

Age at sexual maturity - Swainson's hawk are generally sexually mature
at 2 years of age [31].

Nesting season - The Swainson's hawk arrives on its breeding grounds
later than most raptors [31].  The nesting season generally occurs from
March to October depending on geographic area [14,21,35].  In
California, the Swainson's hawk breeds from late March to mid-August,
with peak activity from late May to late July.  In Nevada, it breeds
from April to October [21].  In Montana, the breeding season is from May
to September [14].
 
Clutch size and incubation - The Swainson's hawk lays two to four eggs,
with two most common [14,21,31,35].  The eggs are incubated for 28 to 35
days [14,21,31].  The Swainson's hawk may lay a replacement clutch if
the first clutch is destroyed [31].

Fledging - Nestlings fledge in 35 to 44 days [21,31].  Fledglings
continue to be fed by the adults and remain within the nesting territory
for 14 to 21 days after fledging; they often return to the nest tree to
roost [21].

Migration - The Swainson's hawk travels in large flocks (sometimes
containing over 100 individuals) from the nesting areas south to their
winter grounds in South America [6,21,31].

Peak fall migration clears the southern plains states and southern Texas
by early October.  The Swainson's hawk arrives in Central America the
last 3 weeks of October to early November; arrival in Argentina is
reported as late November.  Average dates for spring migration of the
Swainson's hawk are mid-March in Panama, the last 3 week of March in
Costa Rica, the last half of March and first week of April in the state
of Veracruz, Mexico, and early April in southern Texas [31].

Longevity - The Swainson's hawk probably seldom lives longer than 16
years [31].

May form premigratory aggregations in summer. Nesting density in suitable habitat varies throughout range from 0.1-1.6 nests per 10 sq km (Bednarz and Hoffman 1988); nests average 1.4-2.4 km apart (see Rothfels and Lein 1983). At one site in California, five nests typically found along a 1 km riparian strip, the nearest nests only 60 meters apart (England et al. 1997). Home ranges during breeding season vary greatly--from 69 to 8718 hectares (reviewed in England et al. 1997). Interspecific territoriality with Red-tailed Hawk in some areas; in other areas may compete with Ferruginous Hawk or be limited by presence of and predation by Great Horned Owl (Palmer 1988).

In California, dispersal distances from natal sites to subsequent breeding sites ranged from 0 to 18 kilometers, mean 8.8 kilometers (Woodbridge et al. 1995). In contrast, none of 697 banded nestlings in Saskatchewan returned to the study area; three were found 190, 200 and 310 kilometers away (Houston and Schmutz 1995).

Range lifespan
Status: wild: 19.5 (high) years.

Average lifespan
Status: wild: 235 months.

Maximum longevity: 19.6 years (wild) Observations: Maximum longevity in banded birds was 19.6 years (Blumstein and Moller 2008).

The Swainson's Hawk starts the breeding season by building nests in March and April. The nest are usually found in trees, shrubs, on the ground, or on top of utility poles. These hawks are mostly mongamous, so a breeding pair may return to a previous nesting site. These birds become highly territorial towards their nest and their mate during this time of the year. When the nest is complete, the female lays 2 to 4 whitish-colored eggs with brown flecks. The male usually helps the female with the incubation, which lasts for about 30 days. The young hatch between March and July, and stay in the nest for another 30 days. While most juveniles migrate the following winter with their parents, there are some groups that do not migrate their first winter (Brown 1996, TPWD 1997).

Average time to hatching: 31 days.

Average eggs per season: 3.

Average age at sexual or reproductive maturity (male)
Sex: male: 730 days.

Average age at sexual or reproductive maturity (female)
Sex: female: 730 days.

Egg dates: mainly April-May in southwestern U.S., California, and Oregon; mainly May-June in central plains states and Canada. Clutch size usually is 2-3. Incubation lasts 34-35 days per egg, almost exclusively by female (male provides food). Young are tended by both adults, leave nest in about 30 days, attain flight at 42-44 days (around 3rd week in July in southwestern U.S.), dependent on parents for 4-4.5 weeks after fledging. First breeds at 2 years. Usually 0.1-0.2 pairs per sq km; average of 1.4-2.4 km between nests. See Bednarz (1988) for information on reproduction in New Mexico. Reported nest density throughout range varies from 0.08-1.61 nests per sq km.

The following is a representative barcode sequence, the centroid of all available sequences for this species.

There are 4 barcode sequences available from BOLD and GenBank.  Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.  See the BOLD taxonomy browser for more complete information about this specimen and other sequences.

Download FASTA File

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 6
Specimens with Barcodes: 6
Species With Barcodes: 1

Information on state-level protected status of animals in the
United States and Canada is available at NatureServe.

Not listed [43]

Year Assessed

Assessor/s

Reviewer/s

Contributor/s

Justification

History

• 2008
  Least Concern
• 2004
  Least Concern

This species of hawk is on the list of Federal Species of Concern, and is also considered threatened by the state of California. The primary cause of this concern is the massive killing of more than 20,000 Swainson's Hawks by pesticides used in the Argentina agricultural areas. In order to help these hawks recover, the use of deadly pesticides by Argentinian farmers must be stopped. Although the farmers are in support of saving the birds, this recovery effort is proving to be a daunting task (Brown 1996, Line 1996).

US Migratory Bird Act: protected

US Federal List: no special status

CITES: appendix ii

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

Canada

Rounded National Status Rank: N4B - Apparently Secure

United States

Rounded National Status Rank: N5B - Secure

Rounded Global Status Rank: G5 - Secure

Reasons: Large breeding range in western and central North America; winters mainly in southern South America; relatively common in some areas, but pesticide use and habitat loss in breeding and nonbreeding range have resulted in declines; recently experienced severe mortality associated with pesticide use in Argentina.

Degree of Threat: B : Moderately threatened throughout its range, communities provide natural resources that when exploited alter the composition and structure of the community over the long-term, but are apparently recoverable

Comments: In California, threats include expansion of cropland unsuitable for foraging (see GHABCOM) and residential and commercial development in former agricultural and grassland areas. In California, Risebrough et al. (1989) concluded that organochlorine contamination of eggs, mortality during migration, and toxic contamination and habitat loss on wintering grounds did not account for a decline of more than 90%. However, widespread use of pesticides and rodenticides throughout the range is cause for concern (California Department of Fish and Game 1990, Kirk and Houston 1995). For example, in early 1996, thousands (perhaps well over 20,000) died as a result of pesticide (Monocrotophos and others) spraying for grasshopper control in croplands in Argentina; those dead were predominantly adults (subadult winter range is unknown); among the dead were hawks banded in Alberta, Saskatchewan, Colorado, Idaho, and California (Geoff Holroyd, Canadian Wildlife Service). Reduced prey populations and shooting possibly are significant threats in Central and South America (Kirk and Houston 1995). See Bednarz and Hoffman (1988) for threats in New Mexico.
Easily disturbed during nesting; often abandons nest if disturbed before the eggs hatch (Biosystems Analysis 1989).

Prescribed fire can be beneficial to Swainson's hawk populations by
enhancing habitat and increasing the prey base [13,25].  Burning in
grasslands where scattered trees are retained benefits Swainson's hawk
populations, particularly in areas where nesting sites are limited.
Prescribed burning plans should strive for creation of maximum
interspersion of openings and edge, with high vegetative diversity.
Habitats should be maintained in a random mosaic.  In most cases,
burning plans must be integrated with proper range management.
Reseeding of perennial grasses as well as rest from livestock grazing
may be necessary to achieve desired goals.  Burning should be deferred
until nesting is completed in areas where impact to breeding Swainson's
hawk may occur [13].

An extensive body of research has been published on fire effects on animals
in semidesert grassland, oak savanna, and Madrean oak woodlands of southeastern
Arizona, including the response of Swainson's hawk to fire. See the Research Project Summary of this work for more information on
Swainson's hawk and more than 100 additional species of birds, small
mammals, grasshoppers, and herbaceous and woody plant species.

More info for the term: density

Swainson's hawk populations have declined over much of their breeding
range due to habitat loss from cultivation, removal of riparian areas,
and removal of shelterbelts [14,31].  Nest site availability may limit
occurrence and breeding density of Swainson's hawk [6].  Schmutz and
others [34] reported that the nesting density of the Swainson's hawk
increased significantly following the erection of 98 artificial nest
platforms in an experimental study plot.  When roads, pipelines, or
other surface facilities are constructed, trees taller than 10 feet (3
m) with a d.b.h. of 2 inches (5 cm) or more should be conserved.  If
destruction of potential nest trees cannot be avoided, they should be
replaced with artificial nest platforms [7].  Within treeless expanses,
constructing artificial nest platforms or planting trees may also benefit
Swainson's hawk populations [6].  Additionally, establishing and
enhancing small wooded areas in the nesting habitat, and protecting
habitat used by the prey base, may benefit the Swainson's hawk [10,39].

The Swainson's hawk is more tolerant of human disturbance than other
hawks and will often nest close to occupied houses [7,14].  However,
intensive human activity in a small area near an active Swainson's hawk
nest would likely result in nest abandonment and breeding failure at
that site [7].

Swainson's hawk is sometimes eaten by people in South America.  Large
numbers are taken from communal roosting areas and killed.  This
activity could have a significant effect on populations that nest in
North America [21].  Additionally, the use of biocides in North,
Central, and South America may have an effect on Swainson's hawk
populations.  The Swainson's hawk may accumulate high pesticide levels
via food-chain concentration [31].

Biological Research Needs: Determine degree of competition with B. jamaicensis.

Global Protection: Few to several (1-12) occurrences appropriately protected and managed

Comments: Probably protected in several federal, state, and provincial parks/refuges, but this may account for relatively few pairs.

Needs: Ensure availability of nesting sites in conjunction with suitable foraging habitat.

The Swainson's Hawk is of special importance to farmers, both in North and South America. Some Swainson's Hawks will live entirely on insects and rodents that it catches in crop fields, thus alleviating some crop destruction for farmers. This species is also important to scientists as they can study the ecological details of its massive migration of over 5,000 miles (Brown 1996, AID 1997, Line 1996).