Vencejo de Chimenea

Identificación

This is a medium-sized swift, measuring from 12 to 15 cm (4.7 to 5.9 in) in length, with a wingspan of 27 to 30 cm (11 to 12 in) and a weight ranging from 17 to 30 g (0.60 to 1.06 oz). The sexes are identical in plumage, though males average slightly heavier than females. The adult's plumage is a dark sooty olive above and grayish brown below, with a slightly paler rump and uppertail covert feathers, and a significantly paler throat. Its upperparts are the most uniformly colored of all the Chaetura swifts, showing little contrast between back and rump. Its beak is black, as are its feet and legs. Its iris is dark brown. Juvenal plumage (that held by juvenile birds) is very similar to that of adults, but with whitish tips to the outer webs of the secondaries and tertials.<p>The chimney swift's wings are slender, curved and long, extending as much as 1.5 in (3.8 cm) beyond the bird's tail when folded. Its wingtips are pointed, which helps to decrease air turbulence (and therefore drag) during flight. Its humerus (the bone in the inner part of the wing) is quite short, while the bones farther out (more distally) along the wing are elongated, a combination which allows the bird to flap very quickly. In flight, it holds its wings stiffly, alternating between rapid, quivering flaps and longer glides. Its flight profile is widely described as a "cigar with wings"—a description first used by Roger Tory Peterson. Although the bird often appears to beat its wings asynchronously during flight, photographic and stroboscopic studies have shown that it beats them in unison. The illusion that it does otherwise is heightened by its very fast and highly erratic flight, with many rapid changes of direction.<p>The legs of the chimney swift, like those of all swifts, are very short. Its feet are small but strong, with very short toes that are tipped with sharp, curved claws. The toes are anisodactyl—three forward, one back—like those of most birds, but the chimney swift can swivel its back toe (its hallux) forward to help it get a better grip. Unlike the legs and feet of most birds, those of the chimney swift have no scales; instead, they are covered with smooth skin.<p>Its tail is short and square, measuring only 4.8 to 5.5 cm (1.90 to 2.15 in) in length. All ten of its tail feathers have shafts which extend as much as 1.3 cm (0.5 in) beyond the vanes, ending in sharp, stiff points. These help the bird to prop itself against vertical surfaces.<p>The chimney swift has large, deep set eyes. These are protected by small patches of coarse, black, bristly feathers, which are located in front of each eye. The swift can change the angle of these feathers, which may help to reduce glare. It is far-sighted and, like some birds of prey, this swift is bifoveal: each eye having both a temporal and a central fovea. These are small depressions in the retina where visual acuity is highest, and help to make its vision especially acute. Like most vertebrates, it is able to focus both eyes at once; however, it is also able to focus a single eye independently.<p>Its bill is very small, with a culmen that measures a mere 5 mm (0.20 in) in length. However, its gape is huge, extending back below its eyes, and allowing the bird to open its mouth very widely. Unlike many insectivorous birds, it lacks rictal bristles at the base of the beak.<p>The chimney swift looks very much like the closely related Vaux's swift, but is slightly larger, with relatively longer wings and tail, slower wingbeats and a greater tendency to soar. It tends to be darker on the breast and rump than the Vaux's swift, though there is some overlap in plumage coloring. It can be as much as 30 percent heavier than Vaux's swift, and its wings, which are proportionately narrower, show a pronounced bulge in the inner secondaries. The chimney swift is smaller, paler and shorter tailed than the black swift. In Central America, it is most similar to Chapman's swift, but it is paler (matte olive rather than glossy black) and has a stronger contrast between its pale throat and the rest of its underparts than does its more uniformly colored relative.

Taxonomía

When Carl Linnaeus first described the chimney swift in 1758, he named it Hirundo pelagica, believing it to be a swallow. This misconception continued well into the 1800s, with ornithologists calling it "American Swallow" (e.g. Mark Catesby) or "Chimney Swallow" (e.g. John James Audubon). In 1825, James Francis Stephens moved this and other small, short-tailed New World swifts to the genus Chaetura, where it has since remained, although some authorities in the 1800s assigned it to a variety of now obsolete genera. It has no subspecies. The chimney swift's closest relative is Vaux's swift. Scientists believe that the two species evolved from a common ancestor that was forced to North America's southeastern and southwestern corners by glacial advances. Separated for millennia by vast ice sheets, the survivors evolved into two species which are still separated by a wide gap across the continent's midsection. It is also closely related to the Chapman's swift; in the past, the three were sometimes treated as a single species.<p>The chimney swift's genus name, Chaetura, is a combination of two Ancient Greek words: chaite, which means "bristle" or "spine", and oura which means "tail". This is an apt description of the bird's tail, as the shafts of all ten tail feathers (rectrices) end in sharp, protruding points. The specific name pelagica is derived from the Greek word pelagikos, which means "of the sea". This is thought to be a reference to its nomadic lifestyle rather than to any reference to the sea, a theory strengthened by the later assignment of the specific name pelasgia (after the nomadic Pelasgi tribe of ancient Greece) to the same species by other ornithologists. Its common name refers to its preferred nesting site and its speedy flight.

Nombre científico

Chaetura pelagica

Autoridad

(Linnaeus, 1758)

Orden

Apodiformes

Familia

Apodidae

Género

Chaetura

Distribución y hábitat

A widespread breeding visitor to much of the eastern half of the United States and the southern reaches of eastern Canada, the chimney swift migrates to South America for the winter. It is a rare summer visitor to the western U.S, and has been recorded as a vagrant in Anguilla, Barbados, Greenland, Jamaica, Portugal, the United Kingdom and the U.S. Virgin Islands. It is found over open country, savanna, wooded slopes and humid forests.<p>The chimney swift's wintering grounds were only discovered in 1944, when bands from birds banded (ringed) in North America were recovered in Peru. An indigenous Peruvian had been wearing the bands as a necklace.

Área de cría

NA : sc, se Canada and c, e USA

Área no reproductiva

w SA

Migración

Migrantes completos

Comportamiento

Patrón migratorio: Migrantes completos.

Conservación

Tendencia poblacional: en declive.

Tendencia poblacional

en declive