The chimney swift is a medium-sized, sooty gray bird with very long, slender wings and very short legs.

[6] 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.

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.

[19] The juvenile plumage (held by young birds for their first few months after fledging) is very similar to that of adults, but with whitish tips to the outer webs of the secondaries and tertials.

[20] The chimney swift's wings are slender, curved and long,[21] extending as much as 1.5 in (3.8 cm) beyond the bird's tail when folded.

[19] 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.

[nb 2] These are small depressions in the retina where visual acuity is highest,[29] and help to make its vision especially acute.

[34] 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.

[34] 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.

It is a rare summer visitor to the western U.S,[36] and has been recorded as a vagrant in Anguilla, Barbados, Greenland, Jamaica, Portugal, the United Kingdom and the U.S. Virgin Islands.

It generally hunts in groups of two or three, migrates in loose flocks of 6–20, and (once the breeding season is over) sleeps in huge communal roosts of hundreds or thousands of birds.

[34] If it is disturbed while at rest, the chimney swift will clap its wings loudly once or twice against its body; it does this either in place, or while dropping down several feet to a lower location.

[44] Researchers estimate that a pair of adults provisioning a nest with three youngsters consume the weight equivalent of at least 5000–6000 housefly-sized insects per day.

[45] Like many bird species, the chimney swift periodically coughs up pellets composed of indigestible bits of prey items.

[47] While most of its food is seized following aerial pursuit, some is gleaned from the foliage of trees; the bird hovers near the ends of branches or drops through upper canopy levels.

However, there are records, particularly during migration periods, of chimney swifts feeding well after dark over brightly lit buildings.

[51] The species shows two-weight peaks each year: one at the start of the breeding season, and a higher one shortly before it begins its migration south in the autumn.

The chimney swift's weight gain before migration is smaller than that of some passerines, suggesting that it must refuel en route at various stopover points.

[52] The chimney swift is a monogamous breeder which normally mates for life, though a small percentage of birds change partners.

[20] Before the arrival of European colonists into North America, the chimney swift nested in hollow trees; now, it uses human-built structures almost exclusively.

[34] In 2010, the International Union for Conservation of Nature changed the chimney swift's status from least concern to near threatened.

[1] The causes of population declines are largely unclear, but may be related to the alteration of the insect community due to pesticide use in the early half of the 20th century.

[74] Populations may have increased historically with the introduction of chimneys to North America by European settlers, providing plentiful nesting opportunities.

[75] Severe storms, such as hurricanes, encountered during migration can seriously impact the chimney's swift's survival rates.

Chimney swifts caught up in 2005's Hurricane Wilma were swept as far north as Atlantic Canada and Western Europe, including six reaching Great Britain, the most ever seen there in a single year.

She commissioned a 28 foot tall tower, of a similar design to a chimney, with ladders and peep holes installed to facilitate observation.

Chimney swifts nested in her tower, and for over fifteen years, she meticulously recorded her observations, filling over 400 pages.