The males and the females of this species 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 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. 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 that allows the bird to flap very quickly. In flight, it holds its wings stiffly, alternating between rapid, quivering flaps and longer glides. 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. Its tail is short and square. All ten of its tail feathers have shafts that 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. 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 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.
Chimney swifts breed in much of the eastern half of the United States and the southern reaches of eastern Canada. They migrate to South America for the winter and are rare summer visitors to the western U.S. These birds are found over open country, ponds, wooded slopes, humid forests, and in suburban and urban areas.
Chimney swifts are gregarious birds and are seldom seen alone. They generally hunt in groups of two or three, migrate in loose flocks of 6-20, and (once the breeding season is over) sleep in huge communal roosts of hundreds or thousands of birds. Like all swifts, they are superb aerialists, and only rarely seen at rest. They drink on the wing, skimming the surface of the water with their beak. They also bathe on the wing, gliding above the surface of a body of water, briefly smacking their breast into the water, then flying off again, shaking their feathers as they go. Chimney swifts are not able to perch upright as most birds do; instead, they cling to vertical surfaces. If they are disturbed while at rest, the birds will clap their wings loudly once or twice against their body; they do this either in place or while dropping down several feet to a lower location. This behavior can result in a loud "thundering" sound if large roosts of the birds are disturbed. The sound is thought to be the bird's way of scaring away potential predators. Chimney swifts forage by day on the wing and remain active into the early evening. However, during migration periods, they may feed well after dark over brightly lit buildings. Most of their food is caught in aerial pursuit but some is gleaned from the foliage of trees; the birds hover near the ends of branches or drop through upper canopy levels. Chimney swifts generally fly quite high, though they descend during cold or rainy weather. These birds have a twittering call, consisting of a rapid series of hard, high-pitched chirps and may also give single chirps.
Chimney swifts are carnivores (insectivores). Most of their food items are flying insects, including various species of flies, ants, wasps, bees, whiteflies, aphids, scale insects, stoneflies, and mayflies. They also eat airborne spiders drifting on their threads.
Chimney swifts are monogamous and generally mate for life, though a small percentage of birds change partners. Breeding birds arrive as early as mid-March in the southern U.S., and as late as mid-May in the Canadian provinces. Pairs perform display flights together, gliding with their wings upraised in a steep "V", and sometimes rocking from side to side. Before the arrival of European colonists into North America, Chimney swifts nested in hollow trees but now, they use only human-built structures. While the occasional nest is still built in a hollow tree (or, exceptionally, in an abandoned woodpecker nest), most are now found in chimneys, or sometimes in airshafts, the dark corners of lightly used buildings, cisterns, or wells. The nest is a shallow bracket made of sticks, which the birds gather in flight, breaking them off trees. The sticks are glued together (and the nest to a vertical surface) with copious amounts of the bird's saliva. The female typically lays 4-5 eggs, though clutch sizes range from 2 to 7. Incubated by both parents, the eggs hatch after 19 days. The chicks are altricial; they are born naked, blind, and helpless. Fledglings leave the nest after about 20 days and creep up vertical walls; they start to fly about 30 days after hatching.
The causes of the Chimney swift 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. Changes in climate can pose another serious threat to these birds. After sudden temperature drops, Chimney swifts sometimes hunt low over concrete roads (presumably following insect prey drawn to the warmer road), where collisions with vehicles become more likely. Severe storms, such as hurricanes, encountered during the migration can also seriously impact the Chimney's swift's survival rates.
According to the IUCN Red List, the total Chimney swift population size is around 15,000,000 individuals or 7,700,000 mature individuals. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List, and its numbers today are decreasing.
