Young common swifts will stay airborne for two to three years, landing only for brief moments to check out possible nesting sites. These birds are able to eat insects caught in mid-air and will drink, sleep and mate while flying.
Common swifts are the only bird that spends this much time on the wing without landing. Even nest building materials are gathered from airborne debris on windy days and are stuck together into a nest with saliva.
The common swift has feet that are ill-adapted to any kind of walking or perching, but can cling to vertical surfaces like cliff faces and the eaves of old houses where they commonly make their nests.
More about swifts:
It is a bit hard to imagine how swifts sleep while still flying.