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Swainson's Hawk
Light-morph adult
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Subclass: Neornithes
Infraclass: Neognathae
Superorder: Neoaves
Order: Falconiformes (but see text)
Family: Accipitridae
Genus: Buteo
Species: B. swainsoni
Binomial name
Buteo swainsoni
Bonaparte, 1838
Synonyms

Buteo swainsonii (lapsus)

The Swainson's Hawk (Buteo swainsoni), is a large buteo hawk of the Falconiformes, sometimes separated in the Accipitriformes like its relatives. This species was named after William Swainson, a British naturalist. It is colloquially known as Grasshopper Hawk or Locust Hawk, as it is very fond of Acrididae (locusts and grasshoppers) and will voraciously eat these insects whenever they are available.

Their breeding habitat is prairie and dry grasslands in western North America. They build a stick nest in a tree or shrub or on a cliff edge. This species is a long-distance migrant, wintering in Argentina; there is a single record of a vagrant from Norway

This species or its immediate predecessor is the ancestor of the Galápagos Hawk, as demonstrated by recent research. The latter diverged from the mainland birds perhaps 300,000 years ago, a very short time in evolution.[1]

Contents

Description

Rufous-morph bird in Hereford, Arizona, on its way to the pampas

The Swainson's Hawk is a slender raptor, slightly smaller than a Red-tailed Hawk (B. jamaicensis). However, the Swainson's Hawk has a slightly longer wingspan and slimmer wings than other soaring hawks. In flight, it hold their wings in a slight dihedral; it tips back and forth slightly while soaring.

There are two main color variations. Over 90 percent of individuals are light-morph; the dark morph is most common in the far west of the range:[2]

  • Light-morph adults are white on the underparts with a dark, reddish "bib" on the chest and a noticeable white throat and face patch. The underwings, seen as the bird soars, have dark linings (leading edge) and pale flight feathers (trailing edge), a pattern unique among North American raptors. The tail is gray-brown with about six narrow dark bands and one wider subterminal band. The upperparts are brown. Juveniles are similar but dark areas have pale mottling and light areas, especially the flanks, have dark mottling. The chest is pale with some darker marks. The subterminal band of the tail is less obvious. Birds in their first spring may have pale heads because of feather wear.
  • Dark-morph birds are dark brown except for a light patch under the tail. There is a rufous variant that is lighter on the underparts with reddish bars. The tails of both these forms resemble those of the light morph.

Range and migration

Swainson's Hawk inhabits North America mainly in the spring and summer and winters in South America. Breeding areas include south-central Alberta, central Saskatchewan, southwestern Manitoba, and west and southern Minnesota. They will breed as far north as east-central Alaska, and southwestern Yukon. Breeding continues south through the eastern parts of Washington and Oregon, locally to the central valley of California, Arizona, New Mexico, and most of Texas. The eastern part of its range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri, northern Illinois, and southwestern Wisconsin.

Swainson's hawk migration route.
30 birds were fitted with satellite tracking devices to produce this map

Small populations winter in southeastern Florida and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico. Individuals reported north of these areas in winter (for example, on Christmas Bird Counts) are almost invariably misidentified buteos of other species. Immature Swainson's hawks winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. It is not known with certainty where most of the adults spend the winter.

Swainson's Hawk appears to feed during the early part of fall migration but not the later stages. It is reported that Central American migrants do not feed or cast pellets. Swainson's Hawk is probably the longest migrant of any North American raptor. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 14,000 miles (22,400 km). Each migration can last at least two months.

They leave the breeding grounds from August to October. Fall migration begins each clear day on which a wind blows in the general direction of travel. Birds gain altitude by soaring in circles on a rising thermal and then set their wings and close their tails as they glide, slowly losing altitude until they find another thermal and rise with it. Thus, waves and small groups are strung out across the sky.

The birds gradually head southwards toward Central America where virtually the entire population funnels through the Isthmus of Panama. Concentrations over locations like Ancon Hill, Balboa, and Panama City are spectacular. In the Andes, it migrates along a narrow corridor and rarely strays off course; for example, it was only recorded in the Serranía de las Quinchas of Colombia – just 100 km or so off its usual migration route – in 2000/2001[3].

In Brazil, migrating birds pass through the western states of Acre and Mato Grosso, while wintering birds may stray to the southern states of Paraná, Rio Grande do Sul and São Paulo. But surprisingly, the occasional Swainson's Hawk – including birds one or two years of age – has also been recorded in the eastern states of Maranhão, Pará, Pernambuco, Piauí and Tocantins, thousands of kilometers away from their usual migration route and wintering grounds and sometimes in mid-summer. This suggests that individuals occasionally become lost during migration, and/or that they may spend a whole year in the tropical regions and range about, rather than just overwintering at one site.[4]

In Uruguay, the first dedicated studies show it to be not uncommon but patchily distributed across the country in winter. Notably, it had been underreported in Flores and Paysandú Departments, where it seems in fact to be a regular visitor. In recent years, the first birds were seen in early November, and some stayed til late February. Numbers increase throughout November and peak in December, when flocks of many dozen roam the open lands. But many stay only for a scant few weeks before leaving again.[5]

Spring migration broadens once the birds have passed through Mexico as they disperse through the breeding range. Migrant groups are noted in the southern U.S. states in March. The earliest Swainson's Hawks arrive in southern Canada in late March, with migration peaking from mid April onwards.

Ecology

Soaring light-morph adult

The habitat of Swainson's Hawk consists of open and semi-open country – deserts, grasslands and prairies – in both its breeding and wintering ranges. It favors wild prairie, hayfields, and pastures over wheat fields and alfalfa fields, which may offer its prey too much cover. It requires elevated perches for hunting and a supply of small mammals such as young ground squirrels as prey for its nestlings. The breeding distribution of the Swainson's hawk is tied very closely to the distribution of various small mammals for this reason. In Saskatchewan, for example, the distribution of Richardson's ground squirrel and the Swainson's hawk are precisely the same.

The Swainson's Hawk will defend its breeding territory from other buteos. Breeding densities may vary from one area to the next but averages one pair per 2.5 square miles (6.5 km²). The average home range estimate for this hawk is 1 to 2 square miles (2.5 to 5 km²). It gathers in groups for feeding and migrating. However, in each case, such gathering is not social, but motivated by good feeding or migrating conditions.

Swainson's Hawk, Red-tailed Hawk (B. jamaicensis) and Ferruginous Hawk (B. regalis) compete for territory, and defend territories against each other. In many parts of the plains these three species nest in the same general area and exploit much the same prey base. Although diets overlap greatly, habitats may not overlap as much. In Oregon, the Swainson's Hawk selects nesting trees having a different configuration than those used by Red-tailed or Ferruginous Hawks. In southern Alberta, different nesting habitats help reduce food competition, with the Swainson's Hawk favoring areas with scattered trees or riparian borders, while Red-tailed Hawks nest in stands of tall trees, and Ferruginous Hawks nest on the open plains.

Reduced reproductive success may result from the Swainson's Hawk's nesting proximity to these two other buteos. The Swainson's Hawk is generally tolerant of people. The bird is attracted to haying, mowing, and plowing operations. House Sparrows, European Starlings, and other small birds may nest in or near a Swainson's Hawk's nest.

In winter quarters, they are far more tolerant, though many birds willl still fend for themselves. In Uruguay, the species likes largely open but broken (with rocks or woods) plains or low hills, where it can be seen to gather in larger groups. Groups of a few dozen birds are not uncommon. Flocks of over one hundred birds have been recorded several times, e.g one that roamed the Cuchilla Marincho region south of Andresito (Flores Department) in mid-late December 2005.[5]

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Hunting and food

Swainson's Hawks soar over open ground with wings held in a dihedral. It occasionally courses low over the ground like a Northern Harrier (Circus cyaneus) or hovers like a Rough-legged Hawk (B. lagopus). It commonly perches on the ground both during migration and on the breeding grounds. During migration, it typically roosts for the night on bare ground with scattered trees, a habit that distinguishes it from fellow long-distance migrants such as the Broad-winged Hawk (B. platypterus), which roosts in closed-canopy woodlands.

These birds patrol open areas or scan for prey from a perch; they may also catch insects in flight. They take advantage of insects turned up by farm equipment or driven out by fire. A hunting Swainson's Hawk will use several strategies. It hunts insects such as dragonflies or dobsonflies while in flight, flapping little as it rides a wind current and stoops upon a fly, grabbing it with its foot and immediately transferring the prey to its bill. It uses a similar strategy to grab individual free-tailed bats from flying streams of bats. Also, when dragonfly hordes are grounded by weather, Swainson's Hawk will stand near groups sheltering from the wind and pluck at individual insects. Swainson's Hawk closely follows both tractors and wild fires for injured or fleeing food. It will also run down insect prey on the ground. Occasionally a hawk will stand still on a dirt bank or elevated mound waiting for prey to appear. It commonly hunts from elevated perches such as telephone poles, stooping on prey when it is sighted.

Common Grasshopper, a favorite foods of Swainson's Hawk

Swainson's Hawk may be largely insectivorous except when nesting. Insect prey commonly taken includes grasshoppers, crickets, and locusts. Pairs bring vertebrate prey to their nestlings, relying heavily upon small mammals such as young ground squirrels, young cottontails, pocket gophers, mice, young jackrabbits, and, at least locally, small birds and other vertebrates including reptiles and amphibians. Birds taken include large birds such as Mallards, and Sage Grouse which may have been injured initially.

Other unusual bird species taken include American Kestrel, and young Short-eared Owls. More typical in size are young Lark Buntings taken at their fledging time. Reptiles, which can comprise large parts of a diet, include snakes such as racers, gopher snakes and striped whipsnakes, and lizards. Amphibians may include tiger salamanders and toads. Swainson's Hawk is an opportunistic feeder which responds quickly to local concentrations of food.

In Argentina, flocks of immature Swainson's Hawks feed on flocks of the migratory darner dragonfly Rhionaeschna bonariensis, following the hordes of insects and feeding mostly on the wing. Local outbreaks of locusts may also be exploited for food by one or more age-classes of birds. The immatures wintering in southern Florida apparently feed upon either insects, mice, or both, when turned up from field plowing. They move from one freshly ploughed field to the next.

There is also some evidence that road-killed birds and animals are also consumed both on the wintering grounds and on the breeding grounds. The species commonly follows tractors and other agricultural equipment during haying or ploughing, where rodents are exposed for the hawks to capture, or insects are uncovered after crop cutting. The Swainson's Hawks have been known to hunt the edges of prairie wildfires.

Reproduction and life span

When Swainson's Hawks arrive at their nesting sites in March or April, they may return to their original nests as these hawks are noted to be monogamous. Research indicates that they have a high degree of mate and territorial fidelity. This is unusual in a long-distance migrant. Seven to 15 days after the birds arrive, the males begin constructing nests on the ground, ledges or in a trees. The nest consists of twigs and grasses and can take up to two weeks to complete. New nests may be constructed, old nests refurbished, or abandoned nests of other species – namely corvids[6] – are refurbished.

The courtship displays of Swainson's Hawk are not well known. One activity involves circling and diving above a potential nest site. The underwings and rump are flashed and the birds call. The display may end with one bird diving to land on the edge of the nest. Copulation occurs mainly in the morning and evening on the dead limbs of trees. The female may assume the receptive position without a prior display. During treading one of the birds calls.

Swainson's Hawks typically nest in isolated trees or bushes, shelterbelts, riparian groves, or around abandoned homesteads. Occasionally, a pair will nest on the ground or on a bank or ledge. Nest trees and bushes include ponderosa pine, Douglas-fir, spruce, cottonwood, domestic poplar, aspen, elm, mesquite, willow, saguaro cactus, and soaptree yuccas. Nests are located from nine to 15 feet above the ground, often in the shaded canopy but near the top of the tree. Nests are flimsy structures, usually smaller than the nests of the Red-tailed Hawk, and often blow down after nesting season.

Juvenile Swainson's Hawk

Clutch size ranges from one to four eggs, but averages two to three. Each egg is elliptical in shape, about 2.25 inches (57 mm) long and 1.8 inches (46 mm) wide. The egg is smooth with fine granulations and the ground color is white, often tinted bluish or greenish. During incubation the shell color quickly wears to dull white. Some eggs are plain; others are lightly marked with spots and blotches of light brown. The incubation period is 34 to 35 days, with the female incubating while the male brings food.

Young Swainson's Hawks are fed small, young mammals. Flight feathers begin to emerge on the young at nine to 11 days. High nestling mortality often occurs when the young are 15 to 30 days old and may be a result of fratricide. The young begin to leave the nest for surrounding branches at 33 to 37 days, fledging occurs at about 38 to 46 days. The fledglings are dependent upon their parents for four to 5 weeks. This species has one brood a year and apparently does not lay replacement clutches.

The oldest wild Swainson's Hawk on record is 24 years. Swainson's Hawks die because of collisions with traffic, illegal shooting, electrocution, and even during severe prairie weather such as hailstorms. Wind storms and hail caused 30 percent nest failure in one study. When sharing a grove with nesting Great Horned Owls, the hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility.

Status and conservation

Injured light-morph Swainson's Hawk recuperating in Zoo Boise

Swainson's Hawk has suffered population declines since the first half of the century and was Blue-listed in the United States from 1972 to 1982. It has since been placed on the National Audubon's List of Special Concern in 1986. It is now listed by the United States Fish and Wildlife Service as a Category 3C candidate. It should be noted that the Swainson's Hawk was removed from the active Federal list because it was found to be more abundant than previously thought; it is not considered a threatened species by the IUCN due to its wide overall range[7]. It remains listed as a threatened species by the California Department of Fish and Game as it has been since 1983.

A major cause of Swainson's Hawk population decline was pesticide use in its wintering grounds of Argentina. Farmers there were using pesticides (DDT and monocrotophos) to control grasshopper and locust infestations, and the Swainson's Hawks were ingesting these pesticides in several different ways, but mainly by gorging themselves on the insects as they lay dying. The U.S. has worked with Argentine farmers to resolve this problem.[8]

Swainson's Hawk has adapted well to grazing and pastureland and seems to be holding its own over much of its breeding range, from northern Mexico to the southern parts of the prairie provinces. However, far western populations, like that of Oregon, and southern California, have drastically declined, often due to habitat loss or incompatible agricultural practices. A possible reason for declines in parts of its range may be agriculturally motivated reductions in populations of both ground squirrels and grasshoppers, major seasonal foods.

Although often nesting close to human activity, some Swainson's Hawks are very easily disturbed at the nest and often desert, especially early in the season. The bird is often quite tame and an easy target for shooters traveling isolated prairie roads. The species may also be affected in ways yet to be understood by some insecticides and herbicides, including those used on its wintering grounds[8].

Observing Swainson's Hawk

One of the best places to view the hawk is in the Snake River Birds of Prey National Conservation Area (NCA) in Idaho. Birds in the NCA are most frequently sighted in mid March, May, and June, in the early morning and evening when they are actively hunting. In April, Swanson's Hawks engage in more sedentary breeding and egg-guarding, and are thus more difficult to spot. In July, rising canyon temperatures make prey scarce, so many birds of prey migrate away.

Footnotes

  1. ^ Bollmer et al. (2005)
  2. ^ Sibley (2000): p.120
  3. ^ Laverde-R. et al. (2005)
  4. ^ Olmos et al. (2006)
  5. ^ a b Azpiroz & Menéndez (2008)
  6. ^ E.g. Common Raven, Black-billed Magpie, and American Crow
  7. ^ BLI (2009)
  8. ^ a b Goldstein et al. (1996)

References

This article incorporates text from the Bureau of Land Management which is in the public domain.
  • Azpiroz, Adrián B. & Menéndez, José L. (2008): Three new species and novel distributional data for birds in Uruguay. Bull. B.O.C. 128(1): 38-56.
  • BirdLife International (BLI) (2009). Buteo swainsoni. In: IUCN 2009. IUCN Red List of Threatened Species. Downloaded on 2010-JAN-07.
  • Bollmer, Jennifer L.; Kimball, Rebecca T.; Whiteman, Noah Kerness; Sarasola, José Hernán & Parker, Patricia G. (2005): Phylogeography of the Galápagos hawk (Buteo galapagoensis): A recent arrival to the Galápagos Islands. Mol. Phylogenet. Evol. 39(1): 237–247. doi:10.1016/j.ympev.2005.11.014 PDF fulltext
  • Goldstein, M.I.; Woodbridge, B.; Zaccagnini, M.E.; Canavelli, S.B. & Lanusse, A. (1996): An assessment of mortality of Swainson's hawks on wintering grounds in Argentina. Journal of Raptor Research 30(2): 106–107.
  • Laverde-R., Oscar; Stiles, F. Gary & Múnera-R., Claudia (2005): Nuevos registros e inventario de la avifauna de la Serranía de las Quinchas, un área importante para la conservación de las aves (AICA) en Colombia [New records and updated inventory of the avifauna of the Serranía de las Quinchas, an important bird area (IBA) in Colombia]. Caldasia 27(2): 247-265 [Spanish with English abstract]. PDF fulltext
  • Olmos, Fábio; Pacheco, José Fernando & Silveira, Luís Fábio (2006): Notas sobre aves de rapina (Cathartidae, Acciptridae e Falconidae) brasileiras [Notes on Brazilian birds of prey]. Revista Brasileira de Ornitologia 14(4): 401-404 [Portuguese with English abstract]. PDF fulltext
  • Sibley, David Allen (2000): The Sibley Guide to Birds. Alfred A. Knopf, New York. ISBN 0-679-45122-6

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