Friday, August 19, 2016

[Ornithology / Behaviour • 2016] Evidence that Birds Sleep in Mid-Flight

Frigatebirds reaches a wingspan of over two meters. They are excellent gliders and can cover several hundred kilometers a day.
photo: B. Voirin    DOI: 10.1038/ncomms12468  

Many birds fly non-stop for days or longer, but do they sleep in flight and if so, how? It is commonly assumed that flying birds maintain environmental awareness and aerodynamic control by sleeping with only one eye closed and one cerebral hemisphere at a time. However, sleep has never been demonstrated in flying birds. Here, using electroencephalogram recordings of great frigatebirds (Fregata minor) flying over the ocean for up to 10 days, we show that they can sleep with either one hemisphere at a time or both hemispheres simultaneously. Also unexpectedly, frigatebirds sleep for only 0.69 h d−1 (7.4% of the time spent sleeping on land), indicating that ecological demands for attention usually exceed the attention afforded by sleeping unihemispherically. In addition to establishing that birds can sleep in flight, our results challenge the view that they sustain prolonged flights by obtaining normal amounts of sleep on the wing.

Figure 1: Measuring the brain state and flight mode of flying frigatebirds.
 (a) Great frigatebird Fregata minor with a head-mounted data logger for recording the electroencephalogram (EEG) from both cerebral hemispheres and head acceleration in three dimensions. A back-mounted GPS logger recorded position and altitude. Photo: B.V. (b) Overhead view of a great frigatebird skull showing (1) the position of the cranial bulge (shaded grey) overlying the hyperpallium of each hemisphere, (2) the position of the epidural electrodes (red dots, EEG; green dot, ground) and (3) the data logger (black rectangle) just posterior to the naso-frontal hinge (arrow). Scale bar is 10 mm. (c) All GPS tracks for individual birds coded with different colours. The Galapagos Islands are outlined with black lines and the study site (Genovesa) is marked by a star. Ocean depth (m) is coded with grey scale. (d) High temporal resolution (1 Hz) 10 min flight trajectory recorded with GPS from a frigatebird (see Supplementary Movie 1 for 3D visualization) showing the circling (soaring) and straight (gliding) flight modes typical of Fregatidae13 (Methods). (e) Altitude, ground speed and airspeed (computed from the GPS data in (d)), tangential and centripetal (radial) low-pass filtered acceleration, and the absolute value of total acceleration (measured by an accelerometer) for the flight in (d). 

Niels C Rattenborg, Bryson Voirin, Sebastian M. Cruz, Ryan Tisdale, Giacomo Dell’Omo, Hans-Peter Lipp, Martin Wikelski and Alexei L. Vyssotski. 2016. Evidence that Birds Sleep in Mid-Flight. Nature Communications. 7: 12468. DOI: 10.1038/ncomms12468 

First evidence of sleep in flight
Birds engage in all types of sleep in flight, but in remarkably small amounts

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