Swainson's hawk
Swainson's hawk | |
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Male in California, USA | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Aves |
Order: | Accipitriformes |
Family: | Accipitridae |
Genus: | Buteo |
Species: | B. swainsoni
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Binomial name | |
Buteo swainsoni Bonaparte, 1838
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Distribution of Swainson's Hawk | |
Synonyms | |
Buteo swainsonii (lapsus) |
Swainson's hawk (Buteo swainsoni) is a large bird species in the Accipitriformes order. This species was named after William Swainson, a British naturalist. It is colloquially known as the 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; it has been recorded as a vagrant in neighboring Chile, in the island countries of the Dominican Republic, and Trinidad and Tobago, and in Norway.[1]
This species or its immediate predecessor is the ancestor of the
Description
Swainson's hawk is a
; it tips back and forth slightly while soaring.There are two main color variations. Over 90% of individuals are light-morph; the dark morph is most common in the far west of the range:[6]
- 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 light linings (leading edge) and dark 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
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. Swainson's hawks mostly winter on the pampas of South America in Argentina, Uruguay, and southern Brazil.[7] The populations of Swainson's hawks breeding in California's Central Valley also winter in western Mexico and central America.[8]
The Swainson's hawk is the second longest migrant of any North American raptor, after arctic nesting Peregrine falcons. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 7,100 mi (11,400 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 (62 mi) or so off its usual migration route – in 2000/2001.[9]
In Brazil, migrating birds pass through the western states of
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 until 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.[11]
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
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 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 Swainson's hawk are precisely the same.[12]
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 sq mi (6.5 km2). The average home range estimate for this hawk is 1 to 2 sq mi (2.6 to 5.2 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, the red-tailed hawk (B. jamaicensis) and the 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, 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 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 Swainson's hawk's nesting proximity to these two other buteos. Swainson's hawk is generally tolerant of people. The bird is attracted to
In winter quarters, they are far more tolerant, though many birds will 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.[11]
Natal Dispersal
Swainson’s hawks have a high natal fidelity, and individuals generally return to areas where they fledged. On average, female natal dispersal (mean distance of 11.1 kilometers) was higher than males (mean distance of 8.3 kilometers) in northern California. Longer natal dispersal distance in females is typical for many bird species, and may aid in inbreeding avoidance. Natal dispersal in the Canadian prairies was significantly higher at 66.7 kilometers, with documented movements as far as 310 kilometers.
Males that fledged in territories with higher primary productivity had lower natal dispersal in northern California, suggesting they were trying to stay closer to more prey-dense habitat.[13]
Hunting and food
Swainson's hawks hunt using various methods. Many still-hunt, watching for prey activity from a perch such as a tree, bush, pylon, telephone pole, hummock or other high object.
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
Swainson's hawks may be largely insectivorous except when nesting. Insect prey commonly taken includes
Other unusual bird species taken include
. 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
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. Wildfires often attract foraging Swainson's hawks, especially grass fires in their South American wintering range. In South American grass fires, the hawks frequently wait around the edges of the fire, picking off not only insects but also vertebrates including nothuras, lizards and snakes.[3]
Reproduction
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 fifteen days after the birds arrive, the males begin constructing nests on the ground, ledges or in a trees. The nest consists of
) — 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
Clutch size ranges from one to four eggs, but averages two to three. Each egg is elliptical in shape, about 2.25 in (57 mm) long and 1.8 in (46 mm) wide. The egg is smooth with fine granulations and the ground color is white, often tinted bluish or greenish. During
Young Swainson's hawks are fed small, young mammals. Flight feathers begin to emerge on the young at 9 to 11 days. High nestling mortality often occurs when the young are 15 to 30 days old and may be a result of
Lifespan
The oldest wild Swainson's hawk on record is 26 years 1 month (Swainson's Hawk 26 years 01 months 07/06/1986 California Local Unknown 07/24/2012 California Saw or photographed neck collar, color band, or other marker (not federal band) while bird was free Alive - Released/Left On Bird). There are a number of cases of Swainson’s hawks living over 20 years old.
Swainson's hawks die because of collisions with
Status and conservation
Swainson's hawk has suffered population declines since the first half of the twentieth century and was
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 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.[17]
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
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.[17]
References
- This article incorporates text from the Bureau of Land Management which is in the public domain.
- ^ . Retrieved 12 November 2021.
- PMID 16376110 – via ResearchGate.
- ^ ISBN 0-7136-8026-1.
- JSTOR 4164111.
- ^ "Swainson's Hawk". All About Birds. Cornell Lab of Ornithology. Retrieved 3 March 2013.
- ISBN 0-679-45122-6.
- PMID 26380528.
- ISSN 0892-1016.
- ^ 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] (PDF). Caldasia (in Spanish). 27 (2): 247–265.
- ^ 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] (PDF). Revista Brasileira de Ornitologia (in Portuguese). 14 (4): 401–404. Archived from the original (PDF) on 2008-12-17.
- ^ a b Azpiroz, Adrián B.; Menéndez, José L. (2008). "Three new species and novel distributional data for birds in Uruguay". Bulletin of the British Ornithologists' Club. 128 (1): 38–56.
- doi:10.1139/z89-366.
- S2CID 84490400.
- ^ Bird Banding Lab (2021-02-22). "Bird Banding Lab Longevity Records".
- ISSN 2562-5667.
- S2CID 216085596.
- ^ a b 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.
External links
- Swainson's hawk: Bird of the Cowboys and Gauchos at the Smithsonian Migratory Bird Center
- About the Swainson's Hawk at The Nature Conservancy
- Swainson's hawk Species Account – Cornell Lab of Ornithology
- Swainson's hawk – Buteo swainsoni – USGS Patuxent Bird Identification InfoCenter
- "Twixt Heaven and Earth" – a documentary from the National Film Board of Canada
- BirdLife species factsheet for Buteo swainsoni
- "Swainson's hawk media". Internet Bird Collection.
- Swainson's hawk photo gallery at VIREO (Drexel University)
- Interactive range map of Buteo swainsoni at IUCN Red List maps
- Audio recordings of Swainson's hawk on Xeno-canto.