Chimney swifts are monogamous; records indicate that some chimney swifts will remain with the same mate for up to eight or nine years.

Mating System: monogamous

Chimney swifts gather together to breed in colonies. Some nesting colonies can be quite large, including thousands of individuals. The exact number of individuals varies depending on the size of the nesting area.

Chimney swifts build their nests in chimneys or hollow trees. The basket-like, half-cup nest is made of sticks secured to the inner wall of a chimney or tree by the hardened saliva of the swifts. The nest is usually placed at least 15.5 m off the ground, but this can vary greatly. A female lays 3 to 7 white, glossy eggs per clutch. Each egg is approximately 2.0 by 1.3 cm. Both parents help incubate the eggs, which means that they will take turns sitting on the nest to keep the eggs warm until they hatch.  Nestlings may leave the nest 14 to 19 days after hatching but the first flight typically occurs 30 days after hatching. Chimney swifts probably reach sexual maturity (have the ability to breed) one year after they have left the nest.

Breeding interval: Chimney swifts breed once yearly, but occasionally have more than one brood per season.

Breeding season: Breeding occurs from May to July.

Range eggs per season: 4 to 7.

Average eggs per season: 5.

Range time to hatching: 19 to 21 days.

Range fledging age: 14 to 19 days.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization

Average eggs per season: 4.

Young chimney swifts are helpless when hatched and are fed by both parents.

Sometimes birds other than the breeding pair will help feed and care for young, a behavior called 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. Records indicate that one colony had more than one-third of the breeding pairs form cooperative groups; there were 22 threesomes and 6 foresomes.

Parental Investment: no parental involvement; altricial ; pre-fertilization; pre-hatching/birth (Protecting); pre-weaning/fledging (Provisioning: Male, Female)