Chimney swifts are monogamous; records indicate that some chimney swifts will remain with the same mate for up to eight or nine years. ()
The breeding season for chimney swifts is from May to July. Nests are placed in the dark in chimneys and occasionally in hollow trees. The basket-like, half-cup nest consists of sticks and is secured to the wall of a chimney by secreted mucilage (saliva). It is usually at least 15.5 m off the ground, but this can vary greatly (Palmer and Fowler, 1975; Whittemore, 1981; Chantler and Driessens, 2000).
Three to seven white, somewhat glossy eggs are laid per clutch (Palmer and Fowler, 1975; Whittemore, 1981; Chantler and Driessens, 2000). Each egg is approximately 2.0 by 1.3 cm. Both parents incubate the eggs (Palmer and Fowler, 1975), and the incubation period is from 19 to 21 days (Chantler and Driessens, 2000). Females will cover the eggs or young at night. Nestlings may leave the nest 14 to 19 days after hatching, but the first flight typically occurs 30 days after hatching (Chantler and Driessens, 2000). Chimney swifts can have more than one brood per season, and will re-nest if the first nest and eggs are destroyed (Whittemore, 1981).
Some nesting colonies can be quite large, made up of thousands of individuals (Chantler and Driessens, 2000). The size of the nesting colony depends on the size of the roosting site; usually there are a few pairs to a few hundred birds in a colony (Dexter, 1969; Chantler and Driessens, 2000). ()
Young chimney swifts are altricial and are fed by both parents.
Sometimes birds other than the breeding pair will help feed and care for young, a behavior called extra-parental cooperation or cooperative breeding. Chimney swifts are known to form cooperative breeding groups of three to four birds. These groups may remain as a nesting unit throughout the season, sharing incubation, brooding, and feeding duties (Dexter, 1952; Chantler and Driessens, 2000). Records indicate that one colony had more than one-third of the breeding pairs form cooperative groups; there were 22 threesomes and 6 foursomes (Dexter, 1952). ()