Wednesday, Apr 23, 2014

Sharp-shinned Hawks

Birds of Prey
Sharp-shinned Hawk

The Sharp-shinned Hawk (Accipiter striatus) is a small hawk. In fact, "sharp-shins" are the smallest to reside in USA and Canada, though some Neotropical species are smaller (notably the aptly named Tiny Hawk).

The taxonomy is far from resolved, with some authorities considering the southern taxa three separate species: White-breasted Hawk (A. chionogaster), Plain-breasted Hawk (A. ventralis) and Rufous-thighed Hawk (A. erythronemius). See taxonomy for further on this.


Distribution / Range

This species is widespread in North America, Central America, South America and the Greater Antilles. Below the distributions of the four groups (see taxonomy) are described as they roughly occur from north to south:

  • The nominate (A. s. striatus) group is widespread in North America, occurring throughout a large part of USA and Canada, except in the ice-covered regions of the far north. Populations in the northern part of the range migrate south and spend the non-breeding season (winter) in southern USA, Mexico and Central America as far south as Panama, with a smaller number spending the winter in the Greater Antilles. Resident populations exists in temperate parts of USA, Canada (in a few coastal regions), Mexico (highlands from Sonora to Oaxaca), Cuba, Hispaniola and Puerto Rico.

Sharp-shinned Hawk (Accipiter striatus)

  • A. (s.) chionogaster (White-breasted Hawk) occurs in highlands from far southern Mexico (Chiapas and Oaxaca), through Honduras, Guatemala and El Salvador, to Nicaragua. It is, as far as known, resident, but some local movements may occur.
  • A. (s.) ventralis (Plain-breasted Hawk) occurs in the coastal mountains of northern Venezuela and Colombia, south through the Andes from western Venezuela, through Colombia, Ecuador and Peru, to central Bolivia. A disjunct population occurs in the Tepuis of southern Venezuela (likely to extend into adjacent parts of Roraima in far northern Brazil, but this remains unconfirmed). It is, as far as known, resident, but some local movements may occur.
  • A. (s.) erythronemius (Rufous-thighed Hawk) is widespread in eastern South America in eastern and southern Brazil, Uruguay, Paraguay, north-eastern Argentina and south-eastern Bolivia. It is, as far as known, resident in some regions and migratory in others. The movements are generally poorly understood, but it only occurs seasonally at some localities in Argentina.
Sharp-shinned Hawk (Accipiter striatus)

Description:

A small Accipiter hawk. Males are 24 to 30 cm (9.5 to 12 in) long, have a wingspan of 52 to 58 cm (20 to 23 in) and weigh from 87 to 114 g (3.1 to 4 oz). As common in Accipiter hawks, females average distinctly larger at a length of 29 to 37 cm (11.5 to 14.5 in), a wingspan of 58 to 68 cm (23 to 27 in) and a weight of 150 to 218 g (5.3 to 7.7 oz). Measurements given here are for the northern group, but they are comparable for the remaining.

Adults have short broad wings and a long square-ended tail banded in blackish and grey (often narrowly tipped white). The remiges (flight feathers - typically only visible in flight) - typically only visible in flight) are whitish barred blackish. Legs yellowish. The hooked bill is black and the cere (soft skin surrounding the nostrils) is yellowish. The remaining plumage varies depending on group:

  • Nominate group: Cap dark and upperparts blue-grey (the former darker). Often, a few more or less random white spots can be seen on the back. Underparts white with rufous or tawny bars. Crissum white. Thighs rufous, but often barred white. The cheeks are tinged rufous (sometimes faint, but generally very distinct in taxa from the Greater Antilles). The irides (eyes) are dark orange to red, but these are yellowish to pale orange in juveniles. Juveniles have dark brownish upperparts with each feather edged rufous, giving a rather scaly appearance. The brown head is streaked whitish, and the whitish underparts are extensively streaked brown.
  • A. (s.) chionogaster (White-breasted Hawk): Resembles the members of the nominate group, but upperparts darker (often appears almost black), thighs whitish-buff and underparts and cheeks entirely white. Juveniles with darker upperparts and distinctly finer streaking below than juveniles of the nominate group.
  • A. (s.) ventralis (Plain-breasted Hawk): Polymorphic. The most common morph (genetic mutation) has dark grey upperparts (often appears almost black) and white underparts variable barred, shaded or mottled with rufous or tawny-buff (extensively marked individuals may appear almost entirely rufous or tawny-buff below). Occasionally, the barring to the lower belly and flanks may appear duskier. The white morph has bluish-grey upperparts (similar to the nominate group), but its underparts are all white except for its rufous thighs. The rare dark morph, the only morph which sometimes lacks rufous thighs, is entirely sooty (occasionally with slight white barring to belly and faint grey bands in tail). The underparts of the females average paler than males of the same morph. The iris is typically yellow (contra illustrations in some books), but individuals (mainly sub-adults?) with a darker iris are occasionally seen. Juveniles have dark brownish or dusky upperparts with each feather typically edged rufous, giving a rather scaly appearance. The underparts are white streaked brown, and the thighs are rufous barred white. Occasionally, juveniles with underparts extensively rufous streaked blackish are seen.
  • A. (s.) erythronemius (Rufous-thighed Hawk): Resembles the nominate group, but upperparts darker, streaking to underparts rufous or dusky, cheeks typically with a clear rufous patch (occasionally lacking almost entirely) and iris yellow (contra illustrations in some books). Juveniles resemble juveniles of the nominate group, but streaking to underparts typically restricted to throat and central underparts, with flanks scaled or barred (often also belly).
Sharp-shinned Hawk (Accipiter striatus)

Identification

  • The northern group is easily mistaken for the slightly larger and lankier Cooper's Hawk, which match the Sharp-shinned in plumage. In flight, the Cooper's, with its longer wings and larger head, is sometimes compared to a "flying cross"; whereas the broader-winged and smaller-headed Sharp-shinned is described as a "flying mallet".

  • A. (s.) chionogaster (White-breasted Hawk) is generally easily recognized by its white underparts. Juv. Bicolored Hawk, juv. Barred Forest Falcon and Collared Forest Falcon generally occur below the altitude of chionogaster, and they have whitish or buff nuchal collars. Juv. Double-toothed Kite and certain Buteo hawks (e.g. Short-tailed Hawk) may show a vaguely similar pattern, but are very differently shaped.

  • A. (s.) ventralis (Plain-breasted Hawk), while itself very variable in plumage, is generally easily recognized by the Accipiter shape and the colour of the underparts. The grey underparts of the Bicolored Hawk are not duplicated by any plumage of ventralis and juv. Bicolored (which may be whitish below) has a nuchal collar. The smaller Tiny Hawk mainly occurs in lowlands, is very small and lacks the rufous thighs of ventralis. The rare dark morph ventralis is arguably the plumage most likely to cause confusion with other species (e.g. White-rumped Hawk, dark morph Collared Forest Falcon and various Buteo hawks), but the yellow eyes and the overall shape means that it too is relatively distinctive.

  • A. (s.) erythronemius (Rufous-thighed Hawk) is distinctive within its range, but commonly confused with the Roadside Hawk (with a very different shape). The Bicolored Hawk is the only other Accipiter within the range of erythronemius which may show yellow eyes and rufous thighs, but it has a different pattern below.

Sharp-shinned vs. Cooper's Hawk


Sharp-shinned Hawk (Accipiter striatus) Habitat / Range:

It occurs in a wide range of woodland and forest types, both dominated by conifers and by various types of broad-leaved trees (especially oaks)

The largest populations of the nominate group (see taxonomy) are thought to occur in the temperate boreal forests, but winter in warmer regions further south (see distribution).

The taxa suttoni, madrensis (both from the nominate group), chionogaster (White-breasted Hawk) and ventralis (Plain-breasted Hawk), are found in upper tropical to temperate highlands; mainly at altitudes of 300-3000 m (990-9850 ft), but occasionally down to near sea-level and up to 4000 m (13100 ft).

The taxon erythronemius (Rufous-thighed Hawk) is found in tropical and subtropical regions; both in lowlands and highlands.


Food and Hunting:

These birds surprise and capture all their prey from cover or while flying quickly through dense vegetation. They are adept at navigating dense thickets.

The great majority of this hawk's prey is small birds, especially various songbirds such as sparrows, wood-warblers and American Robins. Birds caught have ranged in size from a 4.4-g Anna's Hummingbird to a 577-g (1.2-lb) Ruffed Grouse and any bird within this size range is potential prey.

Typically, males will target smaller birds, such as sparrows, and females will pursue larger prey, such as American Robins and flickers. They often pluck the feathers off their prey on a post or other perch.

Rarely, Sharp-shinned Hawks will also eat rodents, lizards, frogs, snakes, and large insects.


Sharp-shinned HawksBreeding:

Sharp-shinned Hawks will construct a stick nest in a large conifer or dense group of deciduous trees. Clutches of 3 to 8 eggs have been recorded, but are usually 4 to 5 eggs. The eggs measure 37.6 x 30 mm (1.48 x 1.18 in) and weigh about 19 g. The eggs are prized by egg-collectors, because they are heavily marked with surprisingly colorful and varied markings.

The incubation period is thought to average at about 30 days. After hatching, the young are brooded for 16 to 23 days by the female, while the male defends the territory and catches food. The young fledge at about a month old and rely on their parents for feeding and protection another 4 weeks.

The nesting sites and breeding behavior of Sharp-shinned Hawks are generally secretive, in order to avoid the predation of larger raptors, such as the Goshawk and the Cooper's Hawk.

While in migration, adults are sometimes preyed on by most of the bird-hunting, larger raptors, especially the Peregrine Falcon. The breeding behavior of the taxa chionogaster (White-breasted Hawk), ventralis (Plain-breasted Hawk) and erythronemius (Rufous-thighed Hawk) are comparably poorly known, but based on the available knowledge they appear to differ little from that of the nominate group.

Juvenile

Conservation

In North America this bird declined in numbers in the 1960s and 1970s, probably as a result of the use of DDT and other pesticides. The population of USA and Canada has rebounded since and might even exceed historical numbers today. This is probably due to the combination of the ban on DDT and the proliferation of backyard birdfeeders in North America which create unnaturally reliable and easy prey for all Accipiters.

Migratory Sharp-shinned Hawks are one of the most numerous raptors recorded at "hawk watches" across the country. An exception is the subspecies from Puerto Rico, venator, which is rare and listed as endangered by the U.S. Fish and Wildlife Service. The remaining resident subspecies from the Greater Antilles, fringilliodes from Cuba and nominate (A. s. striatus) from Hispaniola, are uncommon, local, and, at least in the case of the latter, decreasing. Both ventralis (Plain-breasted Hawk) and erythronemius (Rufous-thighed Hawk) are fairly common (but easily overlooked due to their secretive behavior) and presently considered safe. The situation for chionogaster (White-breasted Hawk) is potentially more problematic due to its limited range, although it, at least locally, remains fairly common.


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