Sharp-shinned hawk


The sharp-shinned hawk is a small hawk, with males being the smallest hawks in the United States and Canada, but with the species averaging larger than some Neotropical species, such as the tiny hawk. The taxonomy is far from resolved, with some authorities considering the southern taxa to represent three separate species: white-breasted hawk, plain-breasted hawk, and rufous-thighed hawk. The American Ornithological Society keeps all four species conspecific.

Taxonomy

The sharp-shinned hawk is sometimes separated into four species, with the northern group retaining both the scientific name and the common name: sharp-shinned hawk. In addition to the nominate taxon, it includes subspecies perobscurus, velox, suttoni, madrensis, fringilloides, and venator. The three remaining taxa, each considered a monotypic species if split, are the white-breasted hawk, plain-breasted hawk and rufous-thighed hawk. The breeding ranges of the groups are entirely allopatric, although the wintering range of the nominate group partially overlaps with the range of chionogaster. This allopatry combined with differences in plumage and, apparently, certain measurements, has been the background for the split, but hard scientific data are presently lacking. Disregarding field guides, most material published in recent years has therefore considered all to be members of a single widespread species – but not without equivocation: Ferguson-Lees et al. say that if they were to make a world list, they would include the three taxa as separate species, and the AOU's comment includes the note "split almost certainly good".
Storer suggested that the southernmost populations within the nominate group were paler below, thus approaching chionogaster. This has also been reflected in recent guides, where A. s. madrensis of southern Mexico is described as being relatively pale below, but if this is a sign of intergradation with chionogaster or a north-south cline which includes both the members of the nominate group and chionogaster remains unclear. In Bolivia, ventralis and erythronemius approach each other, but no evidence of intergradation is known – something that, without actual specimens, also would be hard to prove due to the variability in the plumage of ventralis.

Description

This is a small Accipiter hawk, with males long, with a wingspan of and weight from. As common in Accipiter hawks, females are distinctly larger in size, averaging some 30% longer, and with a weight advantage of more than 50% being common. The female measures in length, has a wingspan of and weighs. The wings measure each, the tail is long and the tarsus is. Measurements given here are for the northern group, but they are comparable for the remaining subspecies. Adults have short broad wings and a medium-length tail banded in blackish and gray with the tip varying among individuals from slightly notched through square to slightly rounded. The remiges are whitish barred blackish. The legs are long and very slender and yellow. The hooked bill is black and the cere is yellowish. The remaining plumage varies depending on group:
.
This species is widespread in North America, Central America, South America and the Greater Antilles. Below, the distributions of the four groups are described as they occur roughly from north to south:
It occurs in a wide range of woodland and forest types, both dominated by conifers and by various types of broad-leaved trees The largest populations of the nominate group are thought to occur in the temperate boreal forests, but winter in warmer regions farther south. The taxa suttoni, madrensis, chionogaster and ventralis, are found in upper tropical to temperate highlands; mainly at altitudes of, but occasionally down to near sea-level and up to. The taxon erythronemius is found in tropical and subtropical regions; both in lowlands and highlands.

Behaviour

Diet

These birds surprise and capture most of their prey from cover or while flying quickly through dense vegetation. They are adept at navigating dense thickets, although this hunting method is often hazardous to the hawk. The great majority of this hawk's prey are small birds, especially various songbirds such as sparrows, wood-warblers, finches, wrens, nuthatches, tits, icterids and thrushes. Birds caught range in size from a Anna's hummingbird to a ruffed grouse and virtually any bird within this size range is potential prey. Typically, males will target smaller birds, such as sparrows and wood-warblers, and females will pursue larger prey, such as American robins and flickers, leading to a lack of conflict between the sexes for prey. These hawks often exploit backyard bird feeders in order to target congregations of ideal prey. 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, the latter typically being dragonflies captured on the wing during the hawk's migration. Bats have occasionally been recorded as a prey of this hawk.
Female with prey.

Reproduction

Sharp-shinned hawks construct a stick nest in a large conifer or dense group of deciduous trees. Clutches of 3 to 8 eggs have been recorded, but 4 to 5 eggs is the typical clutch size. The eggs measure and weigh about. 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 prey. The young fledge at the age of about a month and rely on their parents for feeding and protection another four 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 northern 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, ventralis and erythronemius are comparably poorly known, but based on the available knowledge they appear to differ little from that of the nominate group

Conservation

In North America this species 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, probably due to the combination of the ban on DDT and the proliferation of backyard bird feeders in North America which create unnaturally reliable and easy prey sources. 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, Accipiter striatus venator, which is rare and listed as endangered by the U.S. Fish & Wildlife Service. The remaining resident subspecies from the Greater Antilles, fringilliodes from Cuba and nominate from Hispaniola, are uncommon, local, and, at least in the case of the latter, decreasing. Both ventralis and erythronemius are fairly common and presently considered safe. The situation for chionogaster is potentially more problematic due to its limited range, although it, at least locally, remains fairly common.

Historical material