Swainson's Hawk

Buteo swainsoni

George Jameson
  • ACCIPITRIDAE
  • Hawks, Osprey, Harriers, Eagles, Kites
  • 490,000

Swainson's Hawks have the second longest migration of all raptor species; this species migrates over 10,000 km every spring and fall between its breeding grounds in North America to its wintering grounds in South America. When not breeding, Swainson's have an unusual diet among raptors - they feed mostly on insects! Unfortunately, it is in decline because of habitat destruction, a reduction in its main prey species, and pesticide use.

Identification
Swainson's is a slender buteo with a long tail and pointed wings compared to other buteos. The tail is grayish with narrow, dark bands. The last dark band is the widest and bounded below by a white or buffy band. When seen from below, pale morphs have darker flight feathers than wing-linings. Pale morphs also have a dark breast band or bib. Dark morphs appear to be uniformly colored underneath, except for the tail. All morphs have uniform coloring on their upperparts. Juveniles have a similar underwing pattern as the adult of the same morph and streaking on the underparts with spotting on the breast.

Swainson's is distinguished from White-tailed Hawks which lack the dark breast-band and a white tail with one black band just before the tip. Short-tailed Hawks also lack the dark breast band and have white at the base of their primaries. Broad-winged Hawks have alternating black and white bands on the tail and either an all pale underwing or light flight feathers and dark wing-linings.

Distribution and Population Trends
Swainson's Hawks breed throughout much of the Rocky Mountains and western Great Plains from southern Alberta and Saskatchewan to northern Mexico. They spend the winter mainly in the Pampas of Argentina, but also in other South American countries, southern Mexico, California, and Florida. Their range used to extend into eastern Washington, eastern Oregon, and more of California. Up to 70 pairs breed in the Butte Valley Important Bird Area (IBA) in California. They are also found in the Red Rock Lakes National Wildlife Refuge IBA in Montana and the Deep Springs Valley IBA in California.

Their population took a big hit around the turn of the century, when it went from being repeatedly described as being an abundant and even nuisance species in the late 1800s, to rare and obviously missing from the skies by the 1910s and 1920s. It is now reduced in numbers throughout its range and considered to be declining in Utah, Nevada, and Oregon. Reproductive success in Alberta and Saskatchewan is at a low, most likely caused by a reduced prey base, decay of prairie trees, and plowing of grasslands. Swainson's no longer breed along the southern California coast because it is too highly developed and it no longer occurs in the Mojave Desert. The California population is estimated to be only 90% that of its historical population. Transients to Baja California are decreasingly observed.

Ecology
This hawk inhabits grassland, shrubland, and agricultural areas where it has open areas to forage for its small prey and where roost sites are available. In breeding season, also requires nesting trees, usually trees bordering agricultural fields, in wetland borders, and on abandoned farms. Pair bonds form as soon as the individuals return from the wintering grounds between February and April. Nest-building ensues for another week, and 1-4 eggs are laid soon after. They only have one brood per season. Parents feed their young rodents, rabbits, and reptiles. When adults are not breeding, Swainson's Hawks eat mostly insects, mainly grasshoppers and dragonflies, but also butterflies, moths, and leaf beetles. It forages by soaring over open areas and by searching from perches. It may forage in groups, especially if hunting ground squirrels, grasshoppers, crickets, and bats.

Swainson's Hawks leave their breeding range in August or September and arrive in Argentina in November. Migratory flocks sometimes number from five to ten thousand. Migration from North to South America passes over land and one can imagine the great numbers seen in Central America as the hawks are funneled by the narrowing land mass. Veracruz, Mexico has seen up to 845,000 Swainson's Hawks in one fall. Their high numbers are joined by flocks of Turkey Vultures, Broad-winged Hawks, and Mississippi Kites, making one spectacular sight of migrating raptors. The round-trip migration can cover more than 20,000 km.

Threats
Up until the 1930s, this hawk was shot by ranchers and farmers, even though it preys upon agricultural pests. Their preferred habitat is being converted to urban areas and being planted with woody perennial crops that do not provide good foraging or nesting habitat. The main prey species for hawks in the western Canadian prairie, Richardson's ground-squirrel, is in decline, and is correlated with reduced reproductive rates. Some pairs are intolerant of human disturbance near the nest while nest building and incubating.

It is adapting to annual crop fields like alfalfa and hay fields where prey are abundant and the crops never get too high for foraging. Shelterbelts and tree plantings also provide roosting and sometimes nesting sites. However, large-scale agribusinesses do not have trees, and as small farms are incorporated into larger farms on both its summer and wintering grounds, it is likely that Swainson's will suffer. Pesticide use on alfalfa and sunflower fields in Argentina resulted in the death of some six thousand birds in 1995 and 1996. The alfalfa and sunflower fields were sprayed with organophosphate insecticides to kill grasshopper infestations. Hawks died immediately if they were sprayed while foraging in the fields or within several days after consuming the chemical-ridden grasshoppers.

Conservation
Some conservation plans for this species allow for the loss of some habitat while maintaining existing areas of suitable habitat for Swainson's Hawks. The effects of increased exposure of Swainson's Hawks to urban environments and large-scale agriculture should be monitored. Human disturbance at nesting areas should be restricted during vulnerable periods. Care should be taken to maintain existing nesting trees as the amount of nesting trees limits the amount of available habitat. Trees should be planted in areas that appear to be suitable for Swainson's but that lack nesting trees. Alternative methods should be explored in Argentina to control crop pests, methods that are not toxic to this hawk and other predatory species.

What Can You Do?
Volunteers are crucial to the success of programs that monitor the long-term status of wintering populations of Swainson's Hawk and other bird species. Audubon's Christmas Bird Count (CBC) is one of the longest-running citizen-science monitoring programs in the world and has helped to follow changes in the numbers and distribution of Swainson's Hawk. To learn more about the CBC and how you can participate, visit: http://www.audubon.org/bird/cbc

Audubon's Important Bird Areas program is a vital tool for the conservation of Swainson's Hawks as well as other species. To learn more about the Important Bird Areas program, and how you can help, visit: http://www.audubon.org/bird/iba/.

Information on where Swainson's Hawks occur and in what numbers is vital to conserving the species. Help in monitoring this and other species by reporting your sightings to eBird. A project of Audubon and the Cornell Lab or Ornithology, eBird is the world's first comprehensive online bird monitoring program: http://www.audubon.org/bird/ebird/index.html.

References
England, A. S., M. J. Bechard, and C. S. Houston. 1997. Swainson's Hawk (Buteo swainsoni). In The Birds of North America, No. 265 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.