Search Wildlife
≡
Photo by Jim Parrish
Photo Copyright Jim Parrish
Swainson's Hawk
Buteo swainsoni
NatureServe conservation status
Global (G-rank): G5
State (S-rank): S3B
- Reason: This species occurs throughout Utah as an uncommon summer resident, and it has been reported in the state in winter.
External links
General information
Swainson's hawk, Buteo swainsoni, breeds in much of western North America, from the plains of south-central Canada to northern Mexico. All North American populations migrate south in winter, forming large flocks numbering in the thousands of individuals. Migratory routes avoid large water bodies, and flocks converge over Central America before continuing to foraging grounds primarily in Argentina. Migrating birds may travel more than 5,000 miles in two months. In Utah, this bird is found statewide, though primarily at mid-elevations in the western and northern parts of the state, in shrub and grassland habitats.
Whereas breeding birds feed chiefly on small vertebrate prey, especially ground squirrels, insects are the primary component of the diet during non-breeding periods. Nests are typically in solitary trees or bushes; in the West Desert of Utah, nests are often in junipers. Nests are constructed by both parents of sticks and lined with leafy twigs, grass, and weed stalks. Often nests are used for several years, or the old nests of other hawks or magpies are refurbished. Usually two or three eggs are laid in late spring. Incubation lasts 34 to 35 days, and the female does most of the incubating while the male provides food to the nest. Young are tended by both adults and leave the nest after 30 days. Young are capable of sustained flight after about 42 days, but are dependent on parents for food until they reach about two months of age.
Species range
Hayward et al. (1976) wrote that in Utah this species occurs "throughout the state". There are known occurrences in 22 of the 29 counties in Utah. Nesting occurrences are known in 10 of these counties: Box Elder, Cache, Rich, Salt Lake, Tooele, Wasatch, Uintah, Juab, Grand, and Washington.
Migration
In migration, occurs regularly in most of Middle America, and rarely east along the Gulf Coast to Florida (AOU 1983). In California, migrates March-early May, with a peak in the first half of April, and September-October (Biosystems Analysis, Inc. 1989). Migrants are greatly concentrated as they pass through Panama (mostly March-early April and October-early November; Ridgely and Gwynne 1989). Migrates through Costa Rica late September-November and late February-early May (Stiles and Skutch 1989). In Colombia, flocks of various sizes reported mainly February-March and September-early November (Hilty and Brown 1986). Main northward migration passes through Panama in mid-March, Veracruz in latter half of March and early April, southern Texas and southwstern U.S. chiefly in April (Palmer 1988); fall concentrations and movements occur in August-September in the north, mainly early October in Texas; peak in migration occurs in September in the southwestern U.S.; arrives in Argentina in late November (Palmer 1988). Annual migration flight may be 18,000-27,000 km, encompasses 4 months of the year. See Houston (1990) for information on migrations of Saskatchewan breeders. Migrates in large, often immense, flocks. Migrates over terrain where updrafts provide needed buoyancy for soaring. May roost at night on ground during migration.
Habitat
Nesting sites tend to be absent from high-elevation montane habitats and low-elevation desert flats. Occupied habitat includes sagebrush steppe, juniper stands, grasslands, and agricultural lands (Behle et al. 1985, Hayward et al. 1976), and nests are also frequently in lowland riparian habitat (F. Howe, UDWR, pers. comm.).
Food habits
Vertebrates (mainly mammals) dominate the diet during the breeding season; invertebrates (especially crickets and grasshoppers) are common food at other times and sometimes for nonbreeders in summer. Hawks wintering in Argentina ate mainly dragonflies (Condor 95:475-479, Wilson Bull. 105:365-366). Mammals consumed often include young ground squirrels and pocket gophers. Depending on availability, also eats other small mammals, snakes, lizards, birds, amphibians, and some carrion (e.g., road kills). Hunts for insects on ground; may also catch insects in air. Hunts while soaring or from perch. Does not feed during most of migration (occasional feeding during initial and terminal stages) (Palmer 1988).
Ecology
May form premigratory aggregations in summer. Nesting density in suitable habitat varies throughout range from 0.1-1.6 nests per 10 sq km (Bednarz and Hoffman 1988); nests average 1.4-2.4 km apart (see Rothfels and Lein 1983). At one site in California, five nests typically found along a 1 km riparian strip, the nearest nests only 60 meters apart (England et al. 1997). Home ranges during breeding season vary greatly--from 69 to 8718 hectares (reviewed in England et al. 1997). Interspecific territoriality with Red-tailed Hawk in some areas; in other areas may compete with Ferruginous Hawk or be limited by presence of and predation by Great Horned Owl (Palmer 1988). In California, dispersal distances from natal sites to subsequent breeding sites ranged from 0 to 18 kilometers, mean 8.8 kilometers (Woodbridge et al. 1995). In contrast, none of 697 banded nestlings in Saskatchewan returned to the study area; three were found 190, 200 and 310 kilometers away (Houston and Schmutz 1995).
Reproductive characteristics
Egg dates: mainly April-May in southwestern U.S., California, and Oregon; mainly May-June in central plains states and Canada. Clutch size usually is 2-3. Incubation lasts 34-35 days per egg, almost exclusively by female (male provides food). Young are tended by both adults, leave nest in about 30 days, attain flight at 42-44 days (around 3rd week in July in southwestern U.S.), dependent on parents for 4-4.5 weeks after fledging. First breeds at 2 years. Usually 0.1-0.2 pairs per sq km; average of 1.4-2.4 km between nests. See Bednarz (1988) for information on reproduction in New Mexico. Reported nest density throughout range varies from 0.08-1.61 nests per sq km.
Threats or limiting factors
Hayward et al. (1976), discussing this species in Utah, mentioned that it has suffered recently from "heavy persecution". Because this species feeds heavily on grasshoppers, agricultural pesticide use probably is a threat in Utah as it is elsewhere.
References
- England, A. S., M. J. Bechard, and C. S. Houston. 1997. Swainson’s Hawk (Buteo swainsoni). Birds of North America 265: 28 pp.
- Baicich, P. J., and C. J. O. Harrison. 1997. A guide to the nests, eggs, and nestlings of North American Birds, Second Ed. Academic Press, San Diego. 347 pp.
