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Swainson's Hawk
Buteo swainsoni

Status: Lower risk

Population Trend: Stable.

Other Names: Swainson's Buzzard.

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Buteo swainsoni
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Distribution: Nearctic/Neotropical. Western and central NORTH AMERICA from east-central Alaska and CANADA (British Columbia, Mackenzie) south through UNITED STATES (east to Minnesota) to southern California and northern MEXICO (Chihuahua, Durango); winters mostly in South America, principally in northern ARGENTINA (pampas), southern BRAZIL, PARAGUAY, and URUGUAY, but a few individuals remain in California, MEXICO, and parts of Central America. more....

Subspecies: Monotypic.

Taxonomy: The molecular genetics study of Riesing et al. (2003) indicated that this species and B. galapagoensis are sister taxa. From an analysis of mitochondrial and minisatellite data, Bollmer et al. (2006) found little divergence between Galapagos and Swainson's Hawks and they suggested that the former species is less than 300,000 years old. This lends support to the hypothesis that the Galapagos Hawk originated from "migration dosing" by wayward Swainson's Hawks (Bildstein 2006). Using 19 multi-locus microstellite genotypes, Hull et al. (2008) were able to discern two clusters within United States Swainson's Hawk populations, one in the Central Valley of California and the other in the Great Basin, Great Plains, and southwestern desert region. Whether these differences are due to incipient divergence or the loss of some haplotypes in the much reduced California population is not clear. It may be significant that the Central Valley birds are developing different wintering habits from the rest of the breeding population (Houston 1990, Herzog 1996) and also show a greater frequency of certain plumage morphs than birds breeding in other regions. more....

Movements: Complete long distance, trans-equatorial migrant (Bildstein 2006). The majority of the global population moves back and forth annually between the breeding grounds in North America and the winter range in South America along the Transamerican Flyway (Bildstein and Zalles 2005). A study of spatial segregation of wintering Swainson's Hawks in Argentina, using stable hydrogen isotope analysis, indicated that they do not segregate according to their breeding origins (Sarasola et al. 2007). An increasing number of Swainson's Hawks are wintering in northern areas, e.g., the Central Valley of California (Herzog 1996), coastal Texas (Lockwood and Freeman 2004), coastal western Mexico (Wheeler 2003), and Panama (Jones 2003, 2004). more....

Habitat and Habits: Occurs in open areas, including savannas, grasslands, steppes, and cultivated lands, and also in open woodland, and requires only a small thicket or wooded area for nesting. Swainson's Hawks often forage at grass fires, including prescribed burns, and are attracted to plumes of smoke (Stewart and Smith 2007). more....

Food and Feeding Behavior: Feeds on a wide variety of prey, including small mammals, birds, lizards, snakes, amphibians, and insects, especially grasshoppers. more....

Breeding: Nests are placed in a tree, often in a riparian zone, but also in isolated trees or groves of trees in open areas, including agricultural lands. Cartron et al. (2009) recently reported a high frequency of nesting by Swainson's Hawks on power poles in New Mexico, and pairs occasionally nest on power poles in other parts of the breeding range. The nest is a platform made of large sticks with twigs, weeds, or grass, lined with inner bark and fresh green leaves from surrounding trrees, and some are decorated with flower clusters of willows, lichens, or with down or feathers shed by the hawk (Bent 1937). Clutch size is 2-4 eggs (usually 2), which are pale bluish-white or greeenish-white when fresh and usually lightly spotted with various shades of brown. Both parents incubate, and the incubation period is 28 days (Bent op cit.). Both parents feed the young, and the nestling period at one nest ranged from 28 to 35 days (Cameron 1913). more....

Conservation: Common and widespread in western North America. Categorized as a species of "Least Concern" by BirdLife International. more....

Population Estimates: Based on 1995-1996 data from the Veracruz River of Raptors hawkwatch site, England et al. (1997) estimated the global population at 400,000 individuals, and Rich et al. (2004) put the total at about 460,000 birds. However, based on the totals of migrating birds recorded in Veracruz from 1992-2004, Ruelas Inzunza et al. (2010) arrived at an estimate of 894,000 individuals.

Important References: 
Bent, A.C. 1937. Life histories of North American birds of prey. Order
  Falconiformes (Part 1). U.S. National Museum Bulletin 167.
England, A.S., M.J. Bechard, and C.S. Houston. 1997. Swainson's Hawk
  (Buteo swainsoni). In A. Poole and F. Gill (eds.), The Birds of North
  America no. 265. The Academy of Natural Sciences of Philadelphia,
  Philadelphia, PA., and American Ornithologists' Union, Washington, D.C.
Goldstein, M.L., B. Woodbridge, M.E. Zaccagnini, and S.B. Canavelli. 1996.
  An assessment of mortality of Swainson's Hawks on wintering grounds in
  Argentina. Journal of Raptor Research 30:106-107.
Houston, C.S., and D. Zazelenchuk. 2004. Swainson's Hawk productivity in
  Saskatchewan, 1944-2004. North American Bird Bander 29:174-178.
Kochert, M.N., M.R. Fuller, L.S. Schueck, L. Bond, M.J. Bechard, B.
  Woodbridge, G.L. Holroyd, M.S. Martell, and U. Banasch.
2011. Migration
  patterns, use of stopover areas, and austral summer movements of Swainson's
  Hawks. Condor 113:89-106.
Rodríguez-Estrella, R. 2000. Breeding success, nest-site characteristics,
  and diet of Swainson's Hawks (Buteo swainsoni) in a stable population in
  northern Mexico. Canadian Journal of Zoology 78:1052-1059.
Sarasola, J.H., M.A. Galmes, and M.Á Santillán. 2007. [Ecology and
  conservation of Swainson's Hawk (Buteo swainsoni) in Argentina]. Hornero
  22:173-184. (In Spanish with English summary)
Schmutz, J.K., C.S. Houston, and S.J. Barry. 2001. Prey and reproduction
  in a metapopulation decline among Swainson's Hawks, Buteo swainsoni.
  Canadian Field-Naturalist 115:257-273.
White, C.M. 1994. Swainson's Hawk. P. 181 in del Hoyo, J., A. Elliott, and
  J. Sargatal (eds). Handbook of birds of the world. Vol. 2. New World
  vultures to guineafowl. Lynx Edicions, Barcelona,Spain.
Woodbridge, B., K.K. Finley, and P.H. Bloom. 1995. Reproductive
  performance, age structure, and natal dispersal of Swainson's Hawks in the
  Butte Valley, California. Journal of Raptor Research 29:187-192.

Sites of Interest:
Swainson's Hawk photos.
Swainson's Hawk
Species account with emphasis on Brazil.

Andersen, David
Bechard, Marc J.
Bloom, Peter
Fry, Michael
Fuller, Mark
Galmes, Maximiliano Adrián
Kochert, Michael N.
Lambertucci, Sergio
Lincer, Jeff
Moore, Stan
Olivo Quiroga, Cristian E.
Preston, Charles
Sarasola, José Hernán
Smith, Jeff
Smith, Brian

Last modified: 12/10/2011

Recommended Citation: Global Raptor Information Network. 2021. Species account: Swainson's Hawk Buteo swainsoni. Downloaded from http://www.globalraptors.org on 14 Jun. 2021

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