Q: Which bird can sleep with half its brain while the other half stays awake?
A: A duck. When mallards rest on the water, those on the edge of the flock tend to shut one eye while keeping open the other - presumably to watch for predators, since the open eye is usually the one facing away from the rest of the flock. In 1999 it was shown that when this happens, half the duck's brain is asleep, while the other half is awake. This is called "unihemispheric sleeping", and it has also been observed in other animals, such as lizards.
Source: The research was done by Niels Rattenbourg of Indiana State University - see the press release Birds who sleep with one eye open may be on the look-out for predators.
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