Half-awake to the risk of predation

@article{Rattenborg1999HalfawakeTT,
  title={Half-awake to the risk of predation},
  author={Niels C. Rattenborg and Steven L. Lima and Charles J. Amlaner},
  journal={Nature},
  year={1999},
  volume={397},
  pages={397-398}
}
Birds have overcome the problem of sleeping in risky situations by developing the ability to sleep with one eye open and one hemisphere of the brain awake. Such unihemispheric slow-wave sleep is in direct contrast to the typical situation in which sleep and wakefulness are mutually exclusive states of the whole brain. We have found that birds can detect approaching predators during unihemispheric slow-wave sleep, and that they can increase their use of unihemispheric sleep as the risk of… 
Sleeping under the risk of predation
Sleeping on the wing
TLDR
The ability to interface adaptively with the environment despite sleeping very little challenges commonly held views regarding sleep, and therefore serves as a powerful system for examining the functions of sleep and the consequences of its loss.
Sleeping Birds Do Not Respond to Predator Odour
TLDR
The results suggest that birds are not able to detect predator chemical cues while sleeping, and antipredatory strategies taken before sleep, such as roosting sites inspection, may be crucial to cope with the vulnerability to predation risk while sleeping.
The low-down on sleeping down low: pigeons shift to lighter forms of sleep when sleeping near the ground
TLDR
The similarity of these responses between birds and mammals suggests that REM sleep is influenced by at least some ecological factors in a similar manner in both groups of animals.
Sleep in birds
Unihemispheric sleep in crocodilians?
TLDR
Consistent with observations on unihemispherically sleeping cetaceans and birds, saltwater crocodiles use unilateral eye closure for vigilance, raising the possibility that crocodilians may also sleep with one half of their brain at a time.
Evidence that birds sleep in mid-flight
TLDR
It is established that birds can sleep in flight, and the results challenge the view that they sustain prolonged flights by obtaining normal amounts of sleep on the wing.
...
...