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Swainson's Hawk

Buteo swainsoni ORDER: ACCIPITRIFORMES FAMILY: ACCIPITRIDAE

IUCN Conservation Status: Least Concern

A classic species of the open country of the Great Plains and the West, Swainson’s Hawks soar on narrow wings or perch on fence posts and irrigation spouts. These elegant gray, white, and brown hawks hunt rodents in flight, wings held in a shallow V, or even run after insects on the ground. In fall, they take off for Argentine wintering grounds—one of the longest migrations of any American raptor—forming flocks of hundreds or thousands as they travel.

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At a GlanceHelp

Measurements
Both Sexes
Length
18.9–22 in
48–56 cm
Weight
24.4–48.2 oz
693–1367 g
Relative Size
Larger than a Cooper’s Hawk; smaller than a Ferruginous Hawk.
Other Names
  • Buse de Swainson (French)
  • Aguilucho Langostero, Gavilán Longostero, Aguililla de Swainson (Spanish)

Cool Facts

  • The Swainson’s Hawk initially suffered from a case of mistaken identity, when a specimen collected in Canada in 1827 and illustrated by William Swainson was confused with the common buzzard (Buteo buteo) of Europe. A nephew of Emperor Napoleon eventually corrected the error: in 1832, while working in Philadelphia, French biologist Charles Lucien Bonaparte identified the hawk as a new species and named it after the original illustrator—although he based his own description on a drawing by John James Audubon.
  • Swainson’s Hawk feed their chicks the usual “three r’s” of the North American buteo diet: rodents, rabbits, and reptiles. But when they’re not breeding, the adults switch to a diet made up almost exclusively of insects, especially grasshoppers and dragonflies.
  • Groups of soaring or migrating hawks are called “kettles.” When it comes to forming kettles, Swainson’s Hawks are overachievers: they form flocks numbering in the tens of thousands, often mixing with Turkey Vultures, Broad-winged Hawks, and Mississippi Kites to create a virtual river of migrating birds. Their daytime migrations create a much-anticipated spectacle for birders who in fall and spring form their own flocks at well-known migratory points in the southern U.S., Mexico, and Central America to watch the birds stream by.
  • The oldest known Swainson’s Hawk was a bird first banded in 1986 in California, then recaptured and released in 2011 at 23 years, 11 months old.

Habitat


Grassland

Swainson’s Hawks favor open habitats for foraging. Although much of their native prairie and grassland habitat has been converted to crop and grazing land, these hawks have adjusted well to agricultural settings. You’ll find them searching for prey in hay and alfalfa fields, pastures, grain crops, and row crops, or perched atop adjacent fence posts and overhead sprinkler systems. They rely on scattered stands of trees near agricultural fields and grasslands for nesting sites.

Food


Mammals

Swainson’s Hawks eat mainly mammals and insects. Mammals make up the bulk of the diet during breeding season, when adults prey on ground squirrels, gophers, mice, voles, and rabbits. These opportunistic feeders also eat bats, snakes, lizards, and birds. Regional diets can vary significantly: for instance, in one region near Alberta, Burrowing Owls were a major part of the diet; in Utah, rabbits accounted for more than half the prey taken. When they’re not breeding, Swainson’s Hawks rely almost exclusively on insect prey, especially crickets, grasshoppers, and dragonflies, often catching and eating them on the wing. They also eat butterflies, moths, and beetles.

Nesting

Nesting Facts
Clutch Size
1–5 eggs
Number of Broods
1 broods
Egg Length
2–2.4 in
5.1–6 cm
Egg Width
1.7–1.9 in
4.2–4.7 cm
Incubation Period
34–35 days
Nestling Period
17–22 days
Egg Description
Off-white, often blotched with dark reddish brown or pale purple.
Condition at Hatching
Newly hatched chicks are naked and helpless, and cannot raise their heads. The tiny chicks weigh just under 1.5 ounces (39.4 g).
Nest Description

Although both members of a Swainson’s Hawk pair work on building a new nest, the male brings most of the materials to construct the loose bundle of sticks, twigs, and debris items such as rope and wire. Nest construction can take up to 2 weeks, with the finished nest reaching 2 feet in diameter and over a foot high. The inner bowl measures up to 8 inches around and 2.75 inches deep. Both partners line the bowl with fresh, leafy twigs, grass, hay, weed stalks, or bark; the lining can include cow dung or wool. Swainson’s Hawks may reuse a nest from a previous year, or refurbish a crow, raven, or magpie nest.

Nest Placement

Tree

Male Swainson’s Hawks choose the nest site, usually near the top of a solitary tree or in a small grove of trees along a stream. Pairs often build nests in shelterbelts or other trees located near agricultural fields and pastures where they feed. Nesting trees include willow, black locust, oak, aspen, cottonwood, and conifers. In the southern part of their range Swainson’s Hawks will build nests as little as three feet off the ground in mesquite bushes. On occasion they’ll nest on a power pole or transmission tower.

Behavior


Soaring

Swainson’s Hawks often forage on foot, running after insects and small mammals with wings partly outstretched. They also hover like White-tailed Kites as they scan hayfields and grasslands for prey, and soar low over prairies and pastures when hunting. The hawks have adjusted well to agricultural operations that scare up insects, often catching and eating them on the wing. They’ll also perch on overhead sprinkler rigs or fence posts, and then pounce on rodents fleeing irrigation water. Courting partners perform a “sky dance”: they soar in circles high above the nest site, with the male making steep dives and recoveries before rejoining the female. Breeding birds are aggressive around the nest site and chase off intruders, including Red-tailed Hawks, American Kestrels, Turkey Vultures, and Golden Eagles. Pairs will also dislodge other raptors, including Red-tailed Hawks and White-tailed Kites reusing Swainson’s Hawk nests. In late summer as migration nears, Swainson’s Hawks “flock up” by the thousands to move to their South American wintering grounds. A migrating bird waits for the air to warm, then soars on the rising air currents with wings and tail spread wide. At the top of the thermal the hawk folds its primary feathers back, closes its tails and soars south, using gravity to make distance as it searches for another thermal.

Conservation

status via IUCN

Least Concern

Swainson’s Hawk numbers have been stable overall or slightly increasing since 1966, according to the North American Breeding Bird Survey. However, certain regions have seen declines, particulary in California where only an estimated 10 percent of the original population remains. The BBS shows lesser declines for Manitoba and Kansas. Partners in Flight estimates the global breeding population at 580,000, with 73 percent breeding in the U.S., 20 percent in Canada, and 6 percent in Mexico. They score a 12 out of 20 on the Conservation Concern Score and are not on the 2012 Watch List. Historic declines happened when farmers shot these and other hawks in the belief that they harmed livestock, but shooting hawks (and most other native bird species) is now illegal. More recent declines are due to a loss of prey and nesting sites. Continued consolidation of small farms (which offer shelterbelts of trees suitable for nest sites) into larger agribusiness operations eliminates nesting habitat and threatens breeding populations. The conversion of pastureland to soybean fields in Argentina has led to a loss of winter foraging habitat. Certain pesticides used in Argentina to control grasshoppers (known as monocrotophos and dimethoat) killed thousands of wintering Swainson’s Hawks in the mid 1990s. Since then, an educational campaign and the banning of these pesticides have apparently been successful in reducing mortality, although other pesticides may pose a threat.

Credits

Range Map Help

Swainson
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Migration

Long-distance migrant. Starting in late August and September, nearly the whole population of Swainson’s Hawks migrates from North America to Argentina, a roundtrip of more than 12,000 miles for the northernmost breeders. The migration path narrows as the birds move south and north, creating phenomenal hawk-watching opportunities at known migratory points in Texas, Mexico, and Central America.

Find This Bird

Your best bet for finding Swainson’s Hawks is during summer in open country west of the Mississippi River. They are perch conspicuously on utility poles, fence posts, and isolated trees in areas that otherwise lack such elevated perches. In perch-deprived areas, look for them standing on the ground in grassland or tilled agricultural fields. No other buteo species can be found in large numbers in such situations. During migration, they don’t move along ridgelines or lakeshores nearly as much as do other raptors. Instead, look for large flocks of soaring raptors over open country within their range, especially in April and September. If these are not Turkey Vultures, they are almost certainly Swainson's Hawks. At a few select migration spots including Hazel Bazemore Park (Corpus Christi, Texas) and a few sites in Middle America (Veracruz, Mexico; Kèköldie, Costa Rica; and Panama City, Panama) you can reliably see very large numbers of them passing south in fall. You can also see fairly large numbers in spring at the Santa Ana National Wildlife Refuge hawkwatch in south Texas.

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