American goshawk
American goshawk | |
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Juvenile (left) and adult by Louis Agassiz Fuertes | |
Scientific classification ![]() | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Aves |
Order: | Accipitriformes |
Family: | Accipitridae |
Genus: | Accipiter |
Species: | A. atricapillus
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Binomial name | |
Accipiter atricapillus (Wilson, A, 1812)
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Subspecies | |
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Range of A. atricapillus Resident Non-breeding
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The American goshawk (Accipiter atricapillus) is a species of
Distribution
In North America, they are most broadly found in the western United States, including Alaska, and western Canada. Their breeding range in the western contiguous United States largely consists of the wooded foothills of the Rocky Mountains and many other large mountain ranges from Washington to southern California extending east to central Colorado and westernmost Texas.[4] Somewhat discontinuous breeding populations are found in southeastern Arizona and southwestern New Mexico, thence also somewhat spottily into western Mexico down through Sonora and Chihuahua along the Sierra Madre Occidental as far as Jalisco and Guerrero, their worldwide southern limit as a breeding species.[3][5][6]
The goshawk continues east through much of Canada as a native species, but is rarer in most of the eastern United States, especially the Midwest where they are not typically found outside the Great Lakes region, where a good-sized breeding population occurs in the northern parts of Minnesota, Illinois, Michigan and somewhat into Ohio; a very small population persists in the extreme northeast corner of North Dakota. They breed also in mountainous areas of New England, New York, central Pennsylvania and northwestern New Jersey, sporadically down to extreme northwestern Maryland and northeastern West Virginia.[4] Vagrants have been reported in most of the parts of the United States where they do not breed.[3]
Habitat
![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Accipiter_atricapillus_Kaibab.jpg/220px-Accipiter_atricapillus_Kaibab.jpg)
American and Eurasian goshawks can be found in both
Access to
Description
Similar species
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Coopers-Hawk-on-a-Fence.jpg/170px-Coopers-Hawk-on-a-Fence.jpg)
The juvenile plumage of the species may cause some confusion, especially with other juvenile Accipiter. Unlike other northern Accipiters, the adult American and Eurasian goshawk never has a rusty color to its underside barring.[11] Wing beats of American goshawks are deeper, more deliberate, and on average slower than those of the two other North American Accipiters.[16][17]
American goshawks are sometimes mistaken for species even outside of the genus Accipiter especially as juveniles of each respective species. In North America, four species of buteonine hawk (all four of which are smaller than goshawks to a certain degree) may be confused with them on occasion despite the differing proportions of these hawks, which all have longer wings and shorter tails relative to their size. A species so similar it is sometimes nicknamed the "Mexican goshawk", gray hawk (Buteo plagiatus) juveniles (overlapping with true goshawks in the southwest United States into Mexico) have contrasting face pattern with bold dusky eye-stripes, dark eyes, barred thighs and a bold white "U" on the uppertail coverts. The roadside hawk (Rupornis magnirostris) (rarely in same range in Mexico) is noticeably smaller with paddle-shaped wings, barred lower breast and a buff “U” on undertail coverts in young birds. Somewhat less likely to confuse despite their broader extent of overlap are the red-shouldered hawk (Buteo lineatus) which have a narrow white-barred, dark-looking tail, bold white crescents on their primaries and dark wing edges and the broad-winged hawk (Buteo playpterus) which also has dark wing edges and a differing tapered wing shape. Even wintering gyrfalcon (Falco rusticolus) juveniles have been mistaken for goshawks and vice versa on occasion, especially when observed distantly perched. However, the bulkier, broader headed yet relatively shorter tailed falcon still has many tell-tale falcon characteristics like pointed, longer wings, a brown malar stripe as well as its more extensive barring both above and below.[3]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/The_O%C3%B6logist_for_the_student_of_birds%2C_their_nests_and_eggs_%281911%29_%2814562268417%29.jpg/170px-The_O%C3%B6logist_for_the_student_of_birds%2C_their_nests_and_eggs_%281911%29_%2814562268417%29.jpg)
Juveniles are sometimes confused with the smaller
Taxonomy
The genus Accipiter contains nearly 50 known living species and is the most diverse genus of diurnal raptors in the world. This group of agile, smallish, forest-dwelling hawks has been in existence for possibly tens of millions of years, probably as an adaptation to the explosive numbers of small birds that began to occupy the world's forest in the last few eras. The harriers are the only group of extant diurnal raptors that seem to bear remotely close relation to this genus, whereas buteonines, Old World kites, sea eagles and chanting-goshawks are much more distantly related and all other modern accipitrids are not directly related.[3][11]
Within the genus Accipiter, the American goshawk seems to belong to a
Genetic studies have indicated that the
Subspecies
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/93/Goshawkmale66.jpg/220px-Goshawkmale66.jpg)
- A. a. atricapillus (Dakotas as well as in southeast Alaska thence decreasing mildly along the Pacific in Oregon and California and smallest of all within the race in the Great Basin and Colorado Plateau states (i.e. Nevada, Utah and northern and central Arizona). Conspicuously, wing size did not correspond to variations in body mass and more southerly goshawks were frequently longer winged than the more massive northerly ones.[22][23][24] Male atricapillus goshawk have been found to weigh from 655 to 1,200 g (1.444 to 2.646 lb) and females from 758 to 1,562 g (1.671 to 3.444 lb).[3][25][22][26] The lightest reported mean weights were from goshawks in northern and central Arizona, weighing a mean of 680 g (1.50 lb) in males and 935 g (2.061 lb) while the highest were from a small sample of Alaskan goshawks which weighed some 905 g (1.995 lb) in males and 1,190 g (2.62 lb) in females.[7][27][28] Almost identical mean weights for goshawks as in Alaska were recorded for goshawks from Alberta as well.[22] This race is typically a blue-gray color above with a boldly contrasting black head and broad white supercilia. American goshawks are often grayish below with fine gray waving barring and, compared to most Eurasian goshawks, rather apparent black shaft streaks which in combination create a vermiculated effect that is all-together messier looking than in most Eurasian birds. From a distance, atricapillus can easily appear solidly all-gray from the front.[3] Due to this, the adult goshawk in America is sometimes called the “gray ghost”, a name also somewhat more commonly used for adult male hen harriers.[29] Birds from mainland Alaska tend to be paler overall with more pale flecking than other American goshawks.[11]
- A. a. laingi (Tavernier, 1940) – This insular race is found on Haida Gwaii and Vancouver Island. This subspecies is slightly smaller than the goshawks found on the mainland and is linearly the smallest race on average in North America. The wing chord of males can range from 312 to 325 mm (12.3 to 12.8 in) and that of females is 332 to 360 mm (13.1 to 14.2 in) and is on average nearly 5% smaller than those sampled goshawks from the nearby mainland.[3][30] These goshawks are characteristically darker than mainland goshawks with the black of the crown extending to the interscapulars. The underside is a sootier gray overall.[30][31][32]
- A. a. apache (van Rossem, 1938) – The range of this subspecies extends from southern Arizona and New Mexico down throughout the species' range in Mexico. This subspecies has the longest median wing size of any race, running contrary to Bergmann's rule that northern birds should outsize southern ones in widely distributed temperate species. In males the wing chord ranges from 344 to 354 mm (13.5 to 13.9 in) while in females it ranges from 365 to 390 mm (14.4 to 15.4 in).[3] However, in terms of body mass, it is only slightly heavier than the goshawks found discontinuously somewhat to the north in the Great Basin and the Colorado Plateau and lighter than the heaviest known American goshawks from Alaska, Alberta and Wisconsin despite exceeding the goshawks from these areas in wing size. The weight of 49 males ranged from 631 to 744 g (1.391 to 1.640 lb), averaging 704 g (1.552 lb), while that of 88 females from two studies ranged from 845 to 1,265 g (1.863 to 2.789 lb), averaging 1,006 g (2.218 lb).[25][33] Aside from its overall larger size, apache reportedly averages larger in foot size than most other American goshawks. Birds of this race tend to be darker than other American goshawks aside from the laingi type birds. Due to its shortage of distinct features beyond proportions, this is considered one of the more weakly separated among current separate subspecies, with some authors considering it merely a clinal variation of atricapillus. Even the greater wing size in southern birds follows a trend for the wing chord to increase in size in the south on the contrary to body mass.[3][4][23]
Behavior
Migration
Migratory goshawks in North America may move down to Baja California, Sinaloa and into most of west Texas, but generally in non-irruptive years, goshawks winter no further south than Nebraska, Iowa, Illinois, Indiana, eastern Tennessee and western North Carolina.[3][14][34] Some periodic eruptions to nearly as far as the Gulf of Mexico have been recorded at no fewer than 10 years apart. In one case, a female that was banded in Wisconsin was recovered 1,860 km (1,160 mi) in Louisiana, a first ever record of the species in that state.[35]
Prey availability may primarily dictate the proportion of goshawk populations that migrate and the selection of wintering areas, followed by the presence of snow which may aid prey capture in the short-term but in the long-term is likely to cause higher goshawk mortality.[11][36][37] Showing the high variability of migratory movements, in one study of winter movements of adult female goshawks that bred in high-elevation forests of Utah, about 36% migrated 100 to 613 km (62 to 381 mi) to the general south, 22% migrated farther than that distance, 8.3% migrated less far, 2.7% went north instead of south and 31% stayed throughout winter on their breeding territory.[38] Irruptive movements seem to occur for northern populations, i.e. those of the boreal forests in North America, Scandinavia, and possibly Siberia, with more equal sex ratio of movement and a strong southward tendency of movements in years where prey such as hares and grouse crash.[11] Male young goshawks tend to disperse farther than females, which is unusual in birds, including raptors.[36][39] It has been speculated that larger female juveniles displace male juveniles, forcing them to disperse farther, to the incidental benefit of the species’ genetic diversity. In Cedar Grove, Wisconsin, there were more than twice as many juvenile males than females recorded migrating.[40] At the hawk watch at Cape May Point State Park in New Jersey, few adult males and no adult females have been recorded in fall migration apart from irruptive years, indicating that migration is more important to juveniles.[41] More juveniles were recorded migrating than adults in several years of study from Sweden.[36] In northern Accipiters including the goshawk, there seems to be multiple peaks in numbers of migrants, an observation that suggests partial segregation by age and sex.[42]
Dietary biology
Hunting behavior
In
Prey spectrum
Northern goshawks are usually opportunistic predators, as are most birds of prey. The most important prey species are small to medium-sized mammals and medium to large-sized birds found in forest, edge and scrub habitats.[46]
However, a few prey families dominate the diet in most parts of the range, namely
Birds constitute 47.8% in 33 studies and mammals account for a nearly equal portion of the diet and in some areas rather dominate the food spectrum.[50][51][52] There is some difference in size and type between the prey caught by males and larger females. Prey selection between sexes is more disparate in the more highly dimorphic races from Eurasia than those from North America. The average prey caught by each sex in Arizona was 281.5 g (9.93 oz) and 380.4 g (13.42 oz), respectively, or around a 26% difference.[53]
Corvids
Overall, one prey family that is known to be taken in nearly every part of the goshawk's range is the corvids, although they do not necessarily dominate the diet in all areas. Some 24 species have been reported in the diet of Eurasian and American goshawks. The second most commonly reported prey species in breeding season dietary studies from North America is the 128 g (4.5 oz) Steller's jay (Cyanocitta stelleri). These species were recorded in studies from northwestern Oregon and the Kaibab Plateau of Arizona (where the Steller's made up 37% by number) as the main prey species by number. The conspicuously loud vocalizations, somewhat sluggish flight (when hunting adult or post-fledging individuals) and moderate size of these jays make them ideal for prey-gathering male goshawks.[25][46][54][55][56] In the following areas Corvus species were the leading prey by number: the 457 g (1.008 lb) American crow (Corvus brachyrhynchos) in New York and Pennsylvania (44.8% by number).[25][57][58][59][60] Despite evidence that northern goshawks avoid nesting near common ravens (Corvus corax), the largest widespread corvid (about the same size as a goshawk at 1,040 g (2.29 lb)) and a formidable opponent even one-on-one, they are even known to prey on ravens seldom.[11][25][54][61] Corvids taken have ranged in size from the 72 g (2.5 oz) Canada jay (Perisoreus canadensis) to the raven.[25][62]
Pigeons and doves
Most of American goshawks take pigeons less commonly than Eurasian goshawks.[11][63][64] One exception is in Connecticut where the mourning dove (Zenaida macroura), the smallest known pigeon or dove the goshawk has hunted at 119 g (4.2 oz), was the second most numerous prey species.[25][65]
Gamebirds
The American goshawk is in some parts of its range considered a specialized predator of gamebirds, particularly grouse. All told 33 species of this order have turned up in American and Eurasian goshawks' diet, including most of the species either native to or introduced in North America and Europe. Numerically, only in the well-studied taiga habitats of Canada and Alaska and some areas of the eastern United States do grouse typically take a dominant position. Elsewhere in the range, gamebirds are often secondary in number but often remain one of the most important contributors of prey biomass to nests. With their general ground-dwelling habits, gamebirds tend to be fairly easy for goshawks to overtake if they remain unseen and, if made aware of the goshawk, the prey chooses to run rather than fly. If frightened too soon, gamebirds may take flight and may be chased for some time, although the capture rates are reduced considerably when this occurs. Pre-fledgling chicks of gamebirds are particularly vulnerable due to the fact that they can only run when being pursued.[7][11] There are impressive feats of attacks on other particularly large gamebirds have been reported, in at least one case, successful predation on an estimated 3,900 g (8.6 lb) adult-sized young wild turkey (Meleagris gallopavo) hen by an immature female goshawk weighing approximately 1,050 g (2.31 lb)), although taking adults of much larger-bodied prey like this is considered generally rare, the young chicks and poults of such prey species are likely much more often taken.[66]
Despite reports that grouse are less significant as prey to American goshawks, the 560 g (1.23 lb) ruffed grouse (Bonasa umbellus) is one of the most important prey species in North America (fourth most reported prey species in 22 studies), having been the leading prey species for goshawks in studies from New York, New Jersey and Connecticut (from 12 to 25% of prey selected) and reported as taken in high numbers elsewhere in several parts of their mutual range.[64][65][67][68] The 1,056 g (2.328 lb) sooty grouse (Dendragapus fuliginosus) was reported as the leading prey species in southern Alaska (28.4% by number).[25][49][69] In the boreal forests of Alberta, grouse are fairly important prey especially in winter.[70]
Squirrels
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Martes_americana_Yellowstone_1.jpg/170px-Martes_americana_Yellowstone_1.jpg)
In
Larger tree squirrels are also taken opportunistically, in
Hares and rabbits
Mammals are more important in their diet than in
Other birds
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Colaptes_auratus_FWS.jpg/170px-Colaptes_auratus_FWS.jpg)
Some 21 species of
Corvids as aforementioned are quite important prey. Although they take fewer passerines than other northern Accipiters, smaller types of songbirds can still be regionally important to the diet. This is especially true of the
Status
The breeding range of the American goshawk extends over one-third of North America, a total area of over 30,000,000 km2 (12,000,000 sq mi) for Eurasian and American goshawk. There are a broadly estimated 150,000–300,000 individuals in North America. most western populations at mid-latitudes have approximately 3.6–10.7 pairs/100 km2 (39 sq mi). A total of 107 nesting territories (1991–1996) were located on a 1,754 km2 (677 sq mi) study area on the Kaibab Plateau, AZ, resulting in a density of 8.4 pairs/100 km2 (39 sq mi). The estimated density in Pennsylvania (1.17 pairs/100 km2 (39 sq mi)) suggests that eastern populations may occur at lower densities than western populations, but densities of eastern populations may increase as these populations recover. Typically, populations at far northern latitudes may occur at lower densities than those of southwestern and western populations in North America.
Based on studies from Gotland, Finland and the southwestern United States, annual mortality for adults is 15–21%, however, feather results indicate that annual mortality for adult Eurasian goshawks is up to 7% higher in Europe than American goshawks in North America.[4][11][120][121][122] In many parts of the range, historic populations decreased regionally due to human persecution (especially shooting), disturbance and epidemic loss of habitat, especially during the 19th century and early 20th.[3] Some states, like Pennsylvania, paid $5 bounties on Goshawks in the 1930s.[4]
In the 1950s–1960s declines were increasingly linked with
Seemingly the remaining persistent conservation threat to goshawks, given their seeming overall resilience (at the species level) to both persecution and pesticides, is deforestation. Timber harvests are known to destroy many nests and adversely regional populations.
In North America, several non-governmental conservation organizations petitioned the Department of Interior,
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Historical material
- "Falco atricapillus, Ash-coloured or Black-cap Hawk"; from American Ornithology 2nd edition, volume 1 (1828) by Alexander Wilson and George Ord. Colour plate from 1st edition by A. Wilson.
- John James Audubon. "The Goshawk", Ornithological Biography volume 2 (1834). "Goshawk" (note in Appendix), Ornithological Biography volume 5 (1839). "The Goshawk" (with illustration), Birds of America octavo edition, 1840.