Buteo swainsoni
Swainson’s Hawks are most notable for their long migratory journeys to and from the pampas of Argentina where they spend the Austral summer. During the breeding season in the western United States and Canada they hunt mammals, birds and reptiles in grasslands and farm fields while in South America these hawks primarily eat grasshoppers and dragonflies (England et al. 1997).
DISTRIBUTION. During the 1987-1992 field work seasons of the TBBA project, observers found most breeding evidence for Swainson’s Hawk on the High and Rolling Plains, the northern Edwards Plateau, southern South Texas Brush Country and northeastern Trans-Pecos regions. Breeding evidence was more scattered in other areas especially the Pineywoods (see the region map in Lockwood and Freeman [2004]). Data from the Breeding Bird Survey (BBS) show relative abundances of 1-10 hawks per 40 km (25 mi) route. Relative abundances of this magnitude are found almost exclusively in the Panhandle continuing south to the 31st parallel and east along the Red River almost to the 98th meridian (Sauer et al. 2007).
In Oklahoma breeding is most common in the west, decreasing eastward (Smith 2004), In Colorado breeding is densest on the eastern plans and more scattered through the rest of the state (Preston 1998). Relative abundances of 3-10 are also found in southern Alberta and adjoining areas of Montana and Saskatchewan, western Kansas, eastern and southern New Mexico and Cochise County, Arizona. Other western states and provinces have breeding populations, but mostly at relative abundances of <1 hawk per route (Sauer et al. 2007). The breeding range of this species extends across the United States- Mexico border into the mainland border states (Howell and Webb 1995). The non-breeding season (the Austral summer) is spent primarily on the pampas of Argentina (England et al, 1997, Am. Ornithol. Union 1998).
SEASONAL OCCURRENCE. Swainson’s Hawks arrive in Texas from February through mid-June with most coming between late March and mid-May. The breeding season extends from mid-February to August, based on egg dates from March 1 to July 3 and young in the nest as late as August 20. South- bound migration starts as early as July 25 and continues until November12 with most movement from early August to early November (Oberholser 1974, Lockwood and Freeman 2004).
BREEDING HABITAT. Swainson’s Hawks breed in Texas from near sea level to at least 1800 m (6000 ft) on prairies,,, plain and deserts (Oberholser 1974,). In Arizona atlasers found this hawk breeding most commonly (66%) in grassland and desert habitats (Wise-Gervais 2005) similar to Colorado where grassland and agricultural habitats produced 63% of breeding records (Preston 1998).
Nests of Swainson’s Hawks are commonly placed in solitary trees or bushes, in small; groves or in narrow riparian corridors. Typical tree species include willow, black locust, box elder, juniper, oak and cottonwood. Nests are occasionally placed on power poles or trans- mission towers. The nest, a bulky, unsightly mass of sticks, thistles, sagebrush or brambles, placed near the top of the tree, is lined with fresh leaves and twigs from the nest tree, grass, forb stems and bark. Old Swainson’s Hawk nests are often refurbished (Harrison 1979, England et al. 1997).
In the nest the female usually lays 2 or 3 pale bluish or greenish eggs which fade to white. The eggs, laid at 2 day intervals, are finely marked with browns or unmarked. The incubation which starts with the laying of the first egg, is done by the female and lasts 34-35 days. Young birds take their first flight at 38-46 days after hatching, then remain with their parents for another 22-38 days. The young will not breed until they are >3 years old. Pairs attempt only one clutch per year (Harrison 1979, England et al. 1997).
STATUS. The map in Oberholser (1974) for Swainson’s Hawk is generally similar to the TBBA and BBS maps (Sauer et al. 2007). Lockwood and Freeman (2004) describe the summer status of this hawk in the Panhandle and adjoining areas as uncommon to common and rare in southern and coastal parts of Texas, consistent with BBS relative abundances in Sauer et al. (2007). BBS trend data from 96 routes in this state suggest a slow annual population decline for Texas for the period 1980-2006. This is similar to the North American trend of -0.7% annual population change (Sauer et al. 2007). The use of pesticides in Argentina may be an even greater threat to this hawk than habitat changes in North America.
Text by Robert C. Tweit (2007)
Literature cited.
American Ornithologists’ Union. 1998. Checklist of North American birds, 7th ed. Am, Ornithol. Union, Washington, DC.
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.
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas, University of Texas Press, Austin.
Preston, C. R. 1998. Swainson’s Hawk (Buteo swainsoni). In Colorado Breeding Bird Atlas, pp. 118-119 (H. E. Kingery, ed.), Colorado Bird Atlas Partnership, Denver.
Price, J., S. Droege, and A. Price. 1995. The summer atlas of North American birds. Academic Press, New York.
Sauer, J. R., J. E. Hines, and J. Fallon. 2007. The North American Breeding Bird Survey, results and analysis 1966-2006. Version 7.23.2007. USGS Patuxent Wildlife Research Center, Laurel MD < http://www.mbr-pwrc.usgs.gov/bbs>
Smith, G. A. 2004. Swainson’s Hawk (Buteo swainsoni). In Oklahoma Breeding Bird Atlas, pp. 100-101 (D. L. Reinking, ed.). University of Oklahoma Press, Norman.
Wise-Gervais, C. 2005. Swainson’s Hawk (Buteo swainsoni). In Arizona Breeding Bird Atlas. Pp. 142-143 (T. E. Corman and C. Wise-Gervais, eds.), University of New Mexico Press, Albuquerque.