Can birds sleep while flying?

Related questions

1. Where do Swifts go in winter?
2. What bird can fly the longest?
3. What animal is a swift?
4. What is world's largest bird?
5. Why do swifts fly in circles?
6. What is a flock of swifts called?
7. Do Swifts ever land?
8. Do birds sleep on the wing?
9. What are large birds called?
10. What makes a bird a bird?

Tags

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2. #sleep
3. #suspected
4. #indeed
5. #known
6. #finally
7. #study
8. #years
9. #found
10. #flight
11. #according
12. #researchers
13. #weeks
14. #institute
15. #while
16. #ornithology
17. #birds
18. #however
19. #flying
20. #evidence
21. #species
22. #scientists