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. Back to top
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.Back to top
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.
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.
|Clutch Size:||1-5 eggs|
|Egg Length:||2.0-2.4 in (5.1-6 cm)|
|Egg Width:||1.6-1.9 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).|
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.Back to top
Swainson’s Hawk numbers have been stable overall, with a slight increase between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates the global breeding population at 580,000, with 73% breeding in the U.S., 20% in Canada, and 6% in Mexico. The species rates a 12 out of 20 on the Conservation Concern Score. Swainson's Hawk is not on the 2016 State of North America's Birds' Watch List. Historic declines of this species occurred 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.Back to top
Bechard, Marc J., C. Stuart Houston, Jose H. Saransola and A. Sidney England. 2010. Swainson's Hawk (Buteo swainsoni), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
Crossley, Richard, Jerry Liguori and Brian Sullivan. 2013. The Crossley ID Guide: Raptors: Princeton University Press.
Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.
North American Bird Conservation Initiative. 2014. The State of the Birds 2014 Report. US Department of Interior, Washington, DC, USA.
Partners in Flight. 2017. Avian Conservation Assessment Database. 2017.
Sauer, J. R., D. K. Niven, J. E. Hines, Jr. Ziolkowski, D. J., K. L. Pardieck, J. E. Fallon and W. A. Link. The North American breeding bird survey, results and analysis 1966-2015 (Version 2.07.2017). USGS Patuxent Wildlife Research Center 2017.
Sibley, David Allen. 2014. The Sibley guide to birds, second edition. Alfred A Knopf, New York.