Swifts are the most aerial of birds. They are superficially similar to swallows but are actually not closely related to those passerine species at all; swifts are in the separate order Apodiformes, which they share with the hummingbirds. The treeswifts are closely related to the true swifts, but form a separate family, the Hemiprocnidae.
The resemblances between swifts and swallows are due to convergent evolution reflecting similar life styles based on catching insects in flight.
Many swifts have a characteristic shape, with a short forked tail and very long swept-back wings that resemble a crescent or a boomerang. The flight of some species is characterised by a distinctive "flicking" action quite different from swallows.