maxplanck.nautil.us/article/326/first-evidence-of-sleep-in-flight, posted Oct '18 by peter in bird science
The flight data recorder revealed that frigatebirds sleep in both expected and unexpected ways during flight. During the day the birds stayed awake actively searching for foraging opportunities. As the sun set, the awake EEG pattern switched to a SWS pattern for periods lasting up to several minutes while the birds were soaring. Surprisingly, SWS could occur in one hemisphere at a time or both hemispheres together. The presence of such bihemispheric sleep indicates that unihemispheric sleep is not required to maintain aerodynamic control. Nonetheless, when compared to sleep on land, SWS was more often unihemispheric in flight.