Swifts are small birds (9-25 cm) usually with black or brown plumage. Some species have white on the throat or rump areas and a few species have brighter chestnut or reddish throats. Males and females are monomorphic (look alike) and are capable of high-speed flight. Swifts feed on the wing, and their large gape enables them to catch insects while in flight. Their long, narrow primary feathers and short secondary feathers allow for rapid flight and gliding; because they glide, swifts have small breast muscles relative to other similarly sized birds. Many species have hard tail-feathers with spiny tips to help brace against the walls of their roosting sites.
All swifts have short legs and tiny feet with sharp, curved claws; they cannot perch, but they are able to cling to vertical surfaces such as the cliffs and cave walls that serve as roosting sites. Because swifts use saliva to bind nesting material and attach nests to vertical surfaces, they have large salivary glands that increase in size during the breeding season. Swifts have feathering in front of their eyes; the feathers are thought to reduce glare and protect the eyes. Most species molt after they reach their wintering grounds, although some molt during the breeding season or just prior to migration.
There are two subfamilies of swifts, Cypseloidinae and Apodinae. Species within Cypseloidinae do not use saliva to build nests, have 2 carotid arteries and a primitive palate. Species within Apodinae have a well-developed transpalatine process, one carotid artery, and all but one (needletails Hirundapus) use saliva to build nests.
Sexual Dimorphism: sexes alike