Which bird stays airborne for years at a time?

The common swift. Birds tagged with data loggers were found to stay aloft for ten months without landing once - feeding, drinking, mating and sleeping on the wing, and only coming down to nest. A young swift leaving the nest may not touch anything solid again for two or three years, by which time it has flown a distance comparable to going to the Moon.

Sleeping while flying is the part that sounds impossible. Swifts climb to a couple of kilometres at dusk and again at dawn, and appear to sleep in short bursts on the way down - probably one brain hemisphere at a time, an arrangement well documented in other birds and in dolphins.

The body is built for nothing else. The wings are long and scythe-shaped for cheap gliding, and the legs are so reduced that the old scientific name for the family, Apus, means without feet. A swift cannot walk or take off from flat ground; it clings to vertical surfaces with all four toes turned forwards, and it nests in crevices it can drop out of. Which is why blocking up the eaves of old buildings has been quietly disastrous for them.