A familiar component of the eastern North American avifauna, this small, agile, fast-flying aerial insectivore is easily identified by its characteristic “cigar on wings” profile. It breeds throughout much of southern Canada east of Saskatchewan, south through Texas and all states to the east, and more recently California. Chimney Swifts are most noticeable during migration, when birds numbering in the thousands circle in large tornado-like flocks above roosting chimneys at dusk, and then suddenly descend in ever-narrowing vortices into their depths to spend the night.
While probably more thinly distributed across its breeding range in pre-colonial North America, this swift nested in hollow trees in mature forests until the arrival of European settlers whose chimneys presented a multitude of artificial nesting cavities. This unique commensalism with humans likely contributed to population increases and range expansion as North America settlements expanded. Over the past several decades, however, Chimney Swift populations have been in decline throughout much of their range, likely due to changes in prey abundance, and unknown threats during migration and on the wintering grounds, yet the magnitude and geographic extent of these factors are not well understood. Loss of nesting habitat (suitable chimneys) has often been cited as the cause of population declines, but has so far proven false where investigated.
Chimney Swifts winter in the upper Amazon basin of Peru, Ecuador, Chile, and Brazil but little is known of the biology of these birds while there. They return to North America in March or April in flocks, and pairs of birds soon break away to individual nest sites in chimneys or on the walls of abandoned buildings. There the pair builds a nest of loosely woven twigs against this vertical surface, cemented together with the bird's glue-like saliva. The female lays 4 or 5 eggs then shares the brooding duties with her mate. The eggs hatch in about 19–20 days, and the young fledge about 30 days from hatching. In some instances an unmated helper will assist with the rearing of young. Throughout the breeding season, large flocks of individuals commonly roost together in large chimneys, leading to the misconception that the birds are nesting as a colony. Some nonbreeding birds may even roost in a chimney occupied by a single nesting pair. When the young have fledged, parents and juveniles from several chimneys move to larger staging roosts, and at summer's end they amass in the thousands to migrate to South American wintering areas.
Many aspects of Chimney Swift life history remains unknown, primarily because so much of this bird's day is spent in wide-ranging, fast flight and because its nesting and roosting occur in dark, largely inaccessible sites. Because of the height at which they feed above the ground, the mechanics and ecology of foraging is just beginning to be studied. Some important details of nest life of single pairs were observed in an artificial chimney in Iowa ( Sherman 1952 ). Long-term studies of breeding behavior, nesting biology, growth and development, survivorship, and fidelity to nest sites of many pairs were made in situations where nests were more readily accessible inside old buildings or where nesting sites were relatively close together on accessible rooftops (Dexter Dexter 1946 – Dexter 1992 , Fischer 1958 ). Several studies have pieced together aspects of migratory pathways through the use of banding data ( Calhoun and Dickinson 1942 , Lowery 1943 , Bowman 1952 ). Environmental influences on flocking, roosting, and feeding have been documented ( Michael and Chao 1973 , Zammuto and Franks Zammuto and Franks 1978 – Zammuto and Franks 1981c ). Some physiological discoveries on seasonal salivary-gland changes and use of torpor have been published ( Johnston 1958a , Ramsey 1970 ).
Still, the individual identities of all birds associated with nest sites or roosts were not known, and most young or nests were not followed long enough to document their fates. Thus, substantial gaps in our knowledge of life history and demography remain. However, recent monitoring efforts and ecological studies have begun to improve our understanding of range-wide population declines.