The common swift (Apus apus) is a medium-sized bird, superficially similar to the barn swallow or house martin but somewhat larger, though not stemming from those passerine species, being in the order Apodiformes. The resemblances between the groups are due to convergent evolution, reflecting similar contextual development. The swifts' nearest relatives are the New World hummingbirds and the Southeast Asian treeswifts.
Swifts have very short legs which they use primarily for clinging to vertical surfaces. They never settle voluntarily on the ground, where they would be vulnerable to accidents and predation, and non-breeding individuals may spend up to ten months in continuous flight.
