What bird can sleep while it is flying?

The Swift spends most of its time on the wing; it rarely ever lands, and when it does it prefers to perch on vertical surfaces. A Swift does everything on the wing, including sleeping.

Albatross also sleep while gliding over the sea, opting to sleep while flying than landing on the water where they could be prey for Orca or humans. These birds only ever land for the breeding season.