RESEARCH

SWAINSON'S HAWK (Buteo swainsoni)

SPECIES PROFILE


Description:
The Swainson's Hawk is a medium-sized raptor of western North America, with a length of ~50 cm and an average weight ranging from around 800 grams for males to around 1100 grams for females.

Adults are quite uniformly dark above, and usually have a dark breast band.  The underparts can be quite variable, ranging from mostly white to almost entirely dark.  In flight, they can be recognized by their pointed wingtips, and a distinct contrast in colour between the paler wing lining and the darker primaries and secondaries.  The tail is marked with several fine dark bands, with a broader terminal band.

As in most raptors, females are somewhat larger than males, but there are no reliable differences in plumage.  Juveniles tend to be paler than adults, especially on the back and head.

Similar species:
Closest to the Swainson's Hawk in size and shape is the Red-tailed Hawk.  The dark band on a Red-tailed Hawk is usually lower (on the belly), and the back is not as dark; in flight especially the tail is an excellent field mark to distinguish between the two species.  Other North American buteos (especially Rough-legged, Ferruginous, Red-shouldered, and Broad-winged Hawks) may occasionally be mistaken for Swainson's, but can be reliably separated by experienced observers.



Habitat:
The Swainson's Hawk is primarily a grassland species.  Most of the North American breeding range is in the plains of the western states and prairie provinces, though there are also a few isolated populations in the northwest where they inhabit tundra instead of grasslands.  In winter, the majority of the population is found in the pastures and natural grasslands of the Argentinian plains, though a small number overwinter in south Florida, where they occupy agricultural lands and other open areas.

Range:
The main breeding range of the Swainson's Hawk extends from northern Mexico in the south to central Alberta and Saskatchewan in the north, and from roughly the 100th Meridian in the east to within a couple of hundred kilometres of the Pacific Ocean in the west.  Isolated breeding populations are also known in northeast Alaska, southwest Yukon Territory, and northwest Northwest Territories.  The main migratory route goes through Central America and along the east side of the Andes to northern Argentina. 

Behaviour:
The Swainson's Hawk is territorial during the breeding season, but otherwise quite gregarious, with wintering flocks numbering in the hundreds or even thousands.  They are frequently observed on elevated perches, including fence posts, utility poles and wires, and live or dead trees.  In open areas lacking perches, they will often perch on the ground, typically selecting slightly elevated areas with sparse vegetation.  

Diet:
The Swainson's Hawk's diet in summer is primarily small mammals, but in winter is almost exclusively grasshoppers.  They hunt either from perches, or from the air.  

Conservation issues:
Recent population estimates have been based largely on the migration counts at Veracruz, Mexico, past which the majority of Swainson's Hawks are believed to fly each fall.  In recent years over one million individuals have been counted per season, suggesting that the North American population is at least this large on average.  Though still very numerous, the population is thought to have declined considerably over the years, as the range formerly extended further east and west.

Pesticide induced mortality on the wintering grounds was a serious issue in the mid-1990s.  Beginning in the late 1980s, monocrotophos, a toxic organophosphate, was widely used in Argentina to control grasshopper populations in agricultural areas.  In the winters of 1994-95 and 1995-96, over 24,000 dead Swainson's Hawks were found to be victims of monocrotophos poisoning.  Researchers were quick to identify the cause of mortality, and the Argentinian government responded by banning the use of monocrotophos.  No related deaths have been reported since the winter of 1998-99, and the population appears not to have suffered any long-term decline, thanks to the quick actions by all parties.

Recommended references:
Clark, W.S., and B.K. Wheeler.  2001.  A Field Guide to Hawks of North America.  Revised.  Peterson Field Guide Series, No. 35.  Houghton Mifflin, Boston.

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.).  Academy of Natural Sciences, Philadelphia PA, and American Ornithologists Union, Washington DC.

Goldstein, M.I., T.E. Lacher Jr, B. Woodbridge, M.J. Bechard, S.B. Canavelli, M.E. Zaccagnini, G.P. Cobb, E.J. Scollon, R. Tribolet, and M.J. Hooper.  1999.  Monocrotophos-induced mass mortality of Swainson's hawks in Argentina, 1995-96.  Ecotoxicology 8: 201-214.

Johnsgard, P.A.  1990.  Hawks, eagles, and falcons of North America: biology and natural history.  Smithsonian Institution Press, Washington DC, USA, pp 221-226.

2002- The Migration Research Foundation Inc.