Swainson's Hawk
Encyclopedia
The Swainson's Hawk 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 insect
s 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
.
, 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 holds its 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:
, 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
.
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 Count
s) 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 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.
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.
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 Department
s, 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.
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.
The habitat of Swainson's Hawk consists of open and semi-open country – desert
s, grassland
s 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 mammal
s such as young ground squirrel
s 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 Sparrow
s, European Starling
s, 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 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.
(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 bat
s 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 tractor
s 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.
Swainson's Hawk may be largely insectivorous except when nesting. Insect prey commonly taken includes grasshopper
s, cricket
s, and locust
s. 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 vertebrate
s including reptile
s and amphibian
s. Birds taken include large birds such as Mallard
s, 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.
s and grass
es 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 – 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.
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 hail
storms. Wind storms and hail caused 30 percent nest failure in one study. When sharing a grove
with nesting Great Horned Owl
s, the hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility
.
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. 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.
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 insecticide
s and herbicide
s, including those used on its wintering grounds.
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.
Another good site to see Swanson's is the American Prairie Reserve
in North Eastern Mt.
Buteo
Buteo is a genus of medium to fairly large, wide-ranging raptors with a robust body and broad wings. In the Old World, members of this genus are called "buzzards", but "hawk" is used in North America...
hawk
Hawk
The term hawk can be used in several ways:* In strict usage in Australia and Africa, to mean any of the species in the subfamily Accipitrinae, which comprises the genera Accipiter, Micronisus, Melierax, Urotriorchis and Megatriorchis. The large and widespread Accipiter genus includes goshawks,...
of the Falconiformes
Falconiformes
The order Falconiformes is a group of about 290 species of birds that comprises the diurnal birds of prey. Raptor classification is difficult and the order is treated in several ways.- Classification problems :...
, sometimes separated in the Accipitriformes
Accipitriformes
The Accipitriformes is an order that has been proposed to include most of the diurnal birds of prey: hawks, eagles, vultures, and many others, about 225 species in all. For a long time, the majority view has been to include them with the falcons in the Falconiformes, but some authorities have...
like its relatives. This species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
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
Acrididae
The Acrididae are the predominant family of grasshoppers, comprising some 10,000 of the 11,000 species of the entire suborder Caelifera. The Acrididae are best known because all locusts are of the Acrididae. The subfamily Oedipodinae is sometimes classified as a distinct family Oedipodidae in the...
(locusts and grasshoppers) and will voraciously eat these insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s whenever they are available.
Their breeding habitat is prairie
Prairie
Prairies are considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and grasses, herbs, and shrubs, rather than trees, as the dominant vegetation type...
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
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
, wintering in Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
; there is a single record of a vagrant from Norway
This species or its immediate predecessor is the ancestor of the Galápagos Hawk
Galápagos Hawk
The Galapagos Hawk is a large hawk endemic to the Galapagos Islands.-Physical description:Similar in size to the Red-tailed Hawk and the Swainson's Hawk of North America, the Galapagos Hawk is about 55 cm from beak to tail with a wingspan of 120 cm...
, as demonstrated by recent research. The latter diverged from the mainland birds perhaps 300,000 years ago, a very short time in evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
.
Description
The Swainson's Hawk is a slender raptorBird of prey
Birds of prey are birds that hunt for food primarily on the wing, using their keen senses, especially vision. They are defined as birds that primarily hunt vertebrates, including other birds. Their talons and beaks tend to be relatively large, powerful and adapted for tearing and/or piercing flesh....
, slightly smaller than a Red-tailed Hawk
Red-tailed Hawk
The Red-tailed Hawk is a bird of prey, one of three species colloquially known in the United States as the "chickenhawk," though it rarely preys on standard sized chickens. It breeds throughout most of North America, from western Alaska and northern Canada to as far south as Panama and the West...
(B. jamaicensis). However, the Swainson's Hawk has a slightly longer wingspan and slimmer wings than other soaring hawks.
In flight, it holds its wings in a slight dihedral
Dihedral angle
In geometry, a dihedral or torsion angle is the angle between two planes.The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection...
; 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:
- Light-morph adults are white on the underparts with a dark, reddish "bib" on the chest and a noticeable white throatThroatIn vertebrate anatomy, the throat is the anterior part of the neck, in front of the vertebral column. It consists of the pharynx and larynx...
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 AlbertaAlberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
, central Saskatchewan, southwestern Manitoba
Manitoba
Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...
, and west and southern Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
. They will breed as far north as east-central Alaska, and southwestern Yukon
Yukon
Yukon is the westernmost and smallest of Canada's three federal territories. It was named after the Yukon River. The word Yukon means "Great River" in Gwich’in....
. Breeding continues south through the eastern parts of Washington and Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, locally to the central valley of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
, New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
, and most of Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
. The eastern part of its range includes Minnesota, northwestern Iowa
Iowa
Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...
, most of Nebraska
Nebraska
Nebraska is a state on the Great Plains of the Midwestern United States. The state's capital is Lincoln and its largest city is Omaha, on the Missouri River....
, Kansas
Kansas
Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south...
, and Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...
, northern Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...
, and southwestern Wisconsin
Wisconsin
Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...
.
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
. Individuals reported north of these areas in winter (for example, on Christmas Bird Count
Christmas Bird Count
The Christmas Bird Count is a census of birds in the Western Hemisphere, performed annually in the early Northern-hemisphere winter by volunteer birders...
s) are almost invariably misidentified buteos of other species. Immature Swainson's hawks winter on the pampas of South America in Argentina, Uruguay
Uruguay
Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...
, and southern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. It is not known with certainty where most of the adults spend the winter.
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
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
blows in the general direction of travel. Birds gain altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
by soaring in circles on a rising thermal
Thermal
A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...
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
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
.
The birds gradually head southwards toward Central America where virtually the entire population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
funnels through the Isthmus of Panama
Isthmus of Panama
The Isthmus of Panama, also historically known as the Isthmus of Darien, is the narrow strip of land that lies between the Caribbean Sea and the Pacific Ocean, linking North and South America. It contains the country of Panama and the Panama Canal...
. Concentrations over locations like Ancon Hill
Ancon Hill
Ancon Hill is a steep 654-foot hill which overlooks Panama City, Panama adjacent to the township of Ancón.-Natural Features:It was under U.S. jurisdiction as part of the Panama Canal Zone for much of the 20th century and therefore was never developed like most of the surrounding urbanized parts of...
, Balboa
Balboa, Panama
Balboa is a district of Panama City, located at the Pacific entrance to the Panama Canal.- History :The town of Balboa, founded by the United States during the construction of the Panama Canal, was named after Vasco Núñez de Balboa, the Spanish conquistador credited with discovering the Pacific Ocean...
, and Panama City
Panama City
Panama is the capital and largest city of the Republic of Panama. It has a population of 880,691, with a total metro population of 1,272,672, and it is located at the Pacific entrance of the Panama Canal, in the province of the same name. The city is the political and administrative center of the...
are spectacular. In the Andes
Andes
The Andes is the world's longest continental mountain range. It is a continual range of highlands along the western coast of South America. This range is about long, about to wide , and of an average height of about .Along its length, the Andes is split into several ranges, which are separated...
, it migrates along a narrow corridor and rarely strays off course; for example, it was only recorded in the Serranía de las Quinchas
Serranía de las Quinchas
The Serranía de las Quinchas is an 860 km2 area of tropical rainforest and cloud forest in Colombia. It lies in the mid Magdalena River Valley in the foothills of the Cordillera Oriental of the Colombian Andes. It ranges in altitude from 200 m to 1700 m asl...
of Colombia
Colombia
Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...
– just 100 km or so off its usual migration route – in 2000/2001.
In Brazil, migrating birds pass through the western states of Acre
Acre (state)
Acre is one of the 27 states of Brazil. It is situated in the southwest of the Northern Region, bordering Amazonas to the north, Rondônia to the east, Bolivia to the southeast and the Ucayali Region of Peru to the south and west. It occupies an area of 152,581.4 km2, being slightly smaller...
and Mato Grosso
Mato Grosso
Mato Grosso is one of the states of Brazil, the third largest in area, located in the western part of the country.Neighboring states are Rondônia, Amazonas, Pará, Tocantins, Goiás and Mato Grosso do Sul. It also borders Bolivia to the southwest...
, while wintering birds may stray to the southern states of Paraná
Paraná (state)
Paraná is one of the states of Brazil, located in the South of the country, bordered on the north by São Paulo state, on the east by the Atlantic Ocean, on the south by Santa Catarina state and the Misiones Province of Argentina, and on the west by Mato Grosso do Sul and the republic of Paraguay,...
, Rio Grande do Sul
Rio Grande do Sul
Rio Grande do Sul is the southernmost state in Brazil, and the state with the fifth highest Human Development Index in the country. In this state is located the southernmost city in the country, Chuí, on the border with Uruguay. In the region of Bento Gonçalves and Caxias do Sul, the largest wine...
and São Paulo
São Paulo (state)
São Paulo is a state in Brazil. It is the major industrial and economic powerhouse of the Brazilian economy. Named after Saint Paul, São Paulo has the largest population, industrial complex, and economic production in the country. It is the richest state in Brazil...
. 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
Maranhão
Maranhão is a northeastern state of Brazil. To the north lies the Atlantic Ocean. Maranhão is neighbored by the states of Piauí, Tocantins and Pará. The people of Maranhão have a distinctive accent...
, Pará
Pará
Pará is a state in the north of Brazil. It borders the Brazilian states of Amapá, Maranhão, Tocantins, Mato Grosso, Amazonas and Roraima. To the northwest it also borders Guyana and Suriname, and to the northeast it borders the Atlantic Ocean. The capital is Belém.Pará is the most populous state...
, Pernambuco
Pernambuco
Pernambuco is a state of Brazil, located in the Northeast region of the country. To the north are the states of Paraíba and Ceará, to the west is Piauí, to the south are Alagoas and Bahia, and to the east is the Atlantic Ocean. There are about of beaches, some of the most beautiful in the...
, Piauí
Piauí
Piauí is one of the states of Brazil, located in the northeastern part of the country.Piauí has the shortest coastline of any of the non-landlocked Brazilian states at 66 km , and the capital, Teresina, is the only state capital in the north east to be located inland...
and Tocantins
Tocantins (state)
Tocantins is one of the states of Brazil. . The state was formed in 1988 out of the northern part of Goiás, and construction began on the capital, Palmas, in 1989, in contrast to most of the other cities in the state which date back to the Portuguese colonial period...
, 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.
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
Flores Department
-History and cultural heritage:The department, created by President Máximo Santos, is named after the former Colorado Party leader, Venancio Flores, who was born in Trinidad in the nineteenth century.The department has many sites of prehistoric rock art...
and Paysandú Department
Paysandú Department
Paysandú Department is a department of Uruguay. Its capital is Paysandú. Its name origin is debated but is likely to be of charrua origin.The fertile soils of Paysandú have encouraged much agricultural development. Livestock raising is one of the principal agricultural activities, with cattle...
s, 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.
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
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
s, grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
s and prairies – in both its breeding and wintering ranges. It favors wild prairie, hayfields, and pastures over wheat
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
fields and alfalfa
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
fields, which may offer its prey too much cover. It requires elevated perches for hunting and a supply of small mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s such as young ground squirrel
Squirrel
Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...
s 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
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....
, 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
Territory (animal)
In ethology the term territory refers to any sociographical area that an animal of a particular species consistently defends against conspecifics...
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
Red-tailed Hawk
The Red-tailed Hawk is a bird of prey, one of three species colloquially known in the United States as the "chickenhawk," though it rarely preys on standard sized chickens. It breeds throughout most of North America, from western Alaska and northern Canada to as far south as Panama and the West...
(B. jamaicensis) and Ferruginous Hawk
Ferruginous Hawk
The Ferruginous Hawk , Buteo regalis , is a large bird of prey. It is not a true hawk like sparrowhawks or goshawks, but rather belongs to the broad-winged buteo hawks, known as "buzzards" in Europe...
(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
Tillage
Tillage is the agricultural preparation of the soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shovelling, picking, mattock work, hoeing, and raking...
operations. House Sparrow
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...
s, European Starling
European Starling
The Common Starling , also known as the European Starling or just Starling, is a passerine bird in the family Sturnidae.This species of starling is native to most of temperate Europe and western Asia...
s, 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 will still fend for themselves. In Uruguay
Uruguay
Uruguay ,officially the Oriental Republic of Uruguay,sometimes the Eastern Republic of Uruguay; ) is a country in the southeastern part of South America. It is home to some 3.5 million people, of whom 1.8 million live in the capital Montevideo and its metropolitan area...
, 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
Flores Department
-History and cultural heritage:The department, created by President Máximo Santos, is named after the former Colorado Party leader, Venancio Flores, who was born in Trinidad in the nineteenth century.The department has many sites of prehistoric rock art...
) in mid-late December 2005.
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 HawkBroad-winged Hawk
The Broad-winged Hawk is a small hawk of the genus Buteo. During the summer some subspecies are distributed over eastern North America, as far west as British Columbia and Texas; they then migrate south to winter in the neotropics from Mexico down to southern Brazil. Other subspecies are all-year...
(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
Dragonfly
A dragonfly is a winged insect belonging to the order Odonata, the suborder Epiprocta or, in the strict sense, the infraorder Anisoptera . It is characterized by large multifaceted eyes, two pairs of strong transparent wings, and an elongated body...
or dobsonflies
Dobsonfly
A Dobsonfly is any insect of the subfamily Corydalinae, part of the megalopteran family Corydalidae. There are over 220 species of dobsonflies. Dobsonflies are found throughout the Americas and Asia, as well as South Africa...
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
Beak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
. It uses a similar strategy to grab individual free-tailed bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s 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 tractor
Tractor
A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction...
s 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.
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...
s, cricket
Cricket
Cricket is a bat-and-ball game played between two teams of 11 players on an oval-shaped field, at the centre of which is a rectangular 22-yard long pitch. One team bats, trying to score as many runs as possible while the other team bowls and fields, trying to dismiss the batsmen and thus limit the...
s, and locust
Locust
Locusts are the swarming phase of short-horned grasshoppers of the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory...
s. 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 vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s including reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s and amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s. Birds taken include large birds such as Mallard
Mallard
The Mallard , or Wild Duck , is a dabbling duck which breeds throughout the temperate and subtropical Americas, Europe, Asia, and North Africa, and has been introduced to New Zealand and Australia....
s, and Sage Grouse
Sage Grouse
The Sage Grouse is the largest grouse in North America, where it is known as the Greater Sage-Grouse. Its range is sagebrush country in the western United States and southern Alberta and Saskatchewan, Canada. A population of smaller birds, known in the U.S. as Gunnison Sage-Grouse, were recently...
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
Fledge
Fledge is the stage in a young bird's life when the feathers and wing muscles are sufficiently developed for flight. It also describes the act of a chick's parents raising it to a fully grown state...
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 twigTwig
A twig is a small thin terminal branch of a woody plant. Twigs are critically important in identification of trees, shrubs and vines, especially in wintertime. The buds on the twig are an important diagnostic characteristic, as are the abscission scars where the leaves have fallen away...
s and grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
es 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 – 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
Ponderosa Pine
Pinus ponderosa, commonly known as the Ponderosa Pine, Bull Pine, Blackjack Pine, or Western Yellow Pine, is a widespread and variable pine native to western North America. It was first described by David Douglas in 1826, from eastern Washington near present-day Spokane...
, Douglas-fir
Douglas-fir
Douglas-fir is one of the English common names for evergreen coniferous trees of the genus Pseudotsuga in the family Pinaceae. Other common names include Douglas tree, and Oregon pine. There are five species, two in western North America, one in Mexico, and two in eastern Asia...
, spruce
Spruce
A spruce is a tree of the genus Picea , a genus of about 35 species of coniferous evergreen trees in the Family Pinaceae, found in the northern temperate and boreal regions of the earth. Spruces are large trees, from tall when mature, and can be distinguished by their whorled branches and conical...
, cottonwood, domestic poplar
Poplar
Populus is a genus of 25–35 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar , aspen, and cottonwood....
, aspen, elm
Elm
Elms are deciduous and semi-deciduous trees comprising the genus Ulmus in the plant family Ulmaceae. The dozens of species are found in temperate and tropical-montane regions of North America and Eurasia, ranging southward into Indonesia. Elms are components of many kinds of natural forests...
, mesquite
Mesquite
Mesquite is a leguminous plant of the Prosopis genus found in northern Mexico through the Sonoran Desert and Chihuahuan Deserts, and up into the Southwestern United States as far north as southern Kansas, west to the Colorado Desert in California,and east to the eastern fifth of Texas, where...
, willow
Willow
Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere...
, 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.
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
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
Fratricide
Fratricide is the act of a person killing his or her brother....
. 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
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...
, illegal shooting, electrocution
Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
, and even during severe prairie weather such as hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
storms. Wind storms and hail caused 30 percent nest failure in one study. When sharing a grove
Grove (nature)
A grove is a small group of trees with minimal or no undergrowth, such as a sequoia grove, or a small orchard planted for the cultivation of fruits or nuts...
with nesting Great Horned Owl
Great Horned Owl
The Great Horned Owl, , also known as the Tiger Owl, is a large owl native to the Americas. It is an adaptable bird with a vast range and is the most widely distributed true owl in the Americas.-Description:...
s, the hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...
.
Status and conservation
Blue-listed
Blue-listed species includes any indigenous species or subspecies considered to be vulnerable in their locale. Vulnerable taxa are of special concern because of characteristics that make them particularly sensitive to human activities or natural events. Blue-listed taxa are at risk, but are not...
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. It remains listed as a threatened species by the California Department of Fish and Game
California Department of Fish and Game
The California Department of Fish and Game is a department within the government of California, falling under its parent California Natural Resources Agency. The Department of Fish and Game manages and protects the state's diverse fish, wildlife, plant resources, and native habitats...
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
DDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
and monocrotophos
Monocrotophos
Monocrotophos is an organophosphate insecticide. It is acutely toxic to birds and humans, and for that reason has been banned in the U.S. and many other countries...
) to control grasshopper
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...
and locust
Locust
Locusts are the swarming phase of short-horned grasshoppers of the family Acrididae. These are species that can breed rapidly under suitable conditions and subsequently become gregarious and migratory...
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.
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 insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...
s and herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
s, including those used on its wintering grounds.
Observing Swainson's Hawk
Snake River Birds of Prey National Conservation Area
The Snake River Birds of Prey National Conservation Area has one of the densest populations of nesting raptors. The National Conservation Area is located south of Boise, Idaho along of the Snake River, and is managed by the Bureau of Land Management. The NCA covers...
(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.
Another good site to see Swanson's is the American Prairie Reserve
American Prairie Reserve
The American Prairie Reserve is a private project undertaken by the American Prairie Foundation to create a wildlife conservation area of in northeast Montana through a combination of private and public lands. National Geographic has compared the project to the creation of an American...
in North Eastern Mt.
External links
- Swainson's Hawk Article - Smithsonian Migratory Bird Center
- The Nature Conservancy Birds of Prey: Swainson's Hawk Swainson's Hawk is one of the species benefitting from the Conservancy's Migratory Bird Program. Retrieved 5-SEP-2006.
- Swainson's Hawk Information and Photos - South Dakota Birds and Birding
- Swainson's Hawk Species Account - Cornell Lab of Ornithology
- Swainson's Hawk - Buteo swainsoni - USGS Patuxent Bird Identification InfoCenter
- Watch Twixt Heaven and Earth - a documentary from the National Film Board of Canada