Swifts are monogamous and males and females share equally in nesting and rearing young. In some tropical species pairs will stay together year-round. In other species new pair bonds are formed each year. Nest sites are defended by the nesting pair and fights over nesting sites can last for several hours. Males perform aerial displays and there have been reports of aerial copulation, but no confirmed observations. Mating normally occurs at the nest.
At least one species of swift, chimney swifts (Chaetura pelagica) are cooperative breeders. Breeding pairs can have one or more helpers at the nest. Helpers are usually first-summer non-breeders.
Mating System: monogamous ; cooperative breeder
Breeding in swifts usually coincides with periods of high insect abundance. In the tropics swifts breed during the wet season. In temperate zones breeding occurs in the summer. Swifts living near the equator can breed year-round. Swifts living in areas with long breeding seasons can have two clutches, while those in areas with a short window of time for breeding will have only one. Most swifts are colonial breeders, though some are solitary. Nest sites are usually in dark places such as caves or hollow trees (Vaux’s swifts (Chaetura vauxi) will nest in the cavities made by pileated woodpeckers (Dryocopus pileatus)). Many species of swift have adapted to the human-modified landscape and will nest in man-made structures such as chimneys and under eves of buildings.
Swifts are unique in that they use saliva to glue together nest material and attach their nests to the substrate. Some even use saliva to glue their eggs to the nest. Nests are constructed of a variety of materials: moss, liverwort, feathers and branches. The nests of some swiftlets (Collocaliini) are made entirely of saliva. Cypseloidine swifts build nests with moss and lichen on ledges near or behind waterfalls and will sometimes re-use nests. Clutch size in these species is usually one. Chaeturine swifts build nests out of twigs and use saliva to hold the nest together and glue it to the nesting substrate. Clutch size in these species can be 4 to 5. Apodinae swifts build nests of plant material and feathers on crevices usually on cliffs, but also man-made structures. They are usually colonial nesters; clutch size ranges from 1 to 7, but is commonly 2 to 3. As is common for many colonially nesting species, nest parasites can be plentiful.
Swifts usually begin breeding during their second year. Clutch size varies depending on food quality and availability. Eggs are dull white and range in size from 15.5 by 10 mm to 43 by 28.5 mm. The egg-laying interval is every other day. Hatching is synchronous and incubation lasts from 14 to 32 days. After hatching, the altricial nestlings are brooded for 1 to 2 weeks depending on the weather.
Both swift eggs and nestlings are resistant to cooling, young swifts can go into torpor to conserve energy in cold weather. The weather also has a huge effect on nestling growth since feeding frequency depends on adult foraging success, which in turn depends on the weather. Young swifts have large fat stores and can therefore survive for long periods without being fed.
The nestling period lasts 5 to 8 weeks. This is longer than most similarly-sized birds because the nesting period is extended if food abundance is low. Before fledging, young swifts will perform “wing exercises” as they prepare for their life on the wing. Once they have fledged, young swifts are fully independent and are no longer fed by their parents. Fledgling success ranges from 26 to 96 percent.
Australian swiftlets (Aerodramus terraereginae) have two single-egg clutches per year. The second egg is laid after the first chick hatches and is incubated by the oldest chick. The chick even develops a brood patch similar to an adult’s brood patch. Usually the second chick does not hatch until the first one has fledged. This unique breeding strategy can reduce the length of time it takes to produce two clutches by approximately 3 weeks.
Key Reproductive Features: seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization (Internal )
Both male and female swifts take part in incubating the eggs and feeding the young. Incubation lasts 14 to 32 days. Swifts are altricial and are brooded for 1 to 2 weeks. Chicks stay in the nest for 5 to 8 weeks; nestling growth is highly dependent on the weather since parents have a lower foraging rate in bad weather. Young are fed “food balls” of insects that are held together with saliva; each food ball contains approximately 500 insects. Once chicks fledge, they receive no further parental care.
Parental Investment: altricial ; male parental care ; female parental care