The n-Category Café

Anas platyrhynchos?

It’s the only one i could find reasonably fast but according to the Wikipedia article on Unispheric slow-wave sleep there ought to be more.

Googling got me to the following site: Phylogeny of sleep: about the database.

According to which : “Terrestrial, placental mammals may be the only animals with bi-hemispheric sleep (i.e., the same type of sleep occurring simultaneously in both hemispheres). Marine mammals, birds, and some reptiles all display some form of unihemispheric sleep (i.e., one form of sleep occurring in a single hemisphere at a time).”

That kind of baffled me. I had already found that the 17% of the sleep of the glaucous winged gull was of the usws kind according to: Evolution of sleep. Now i think i know what that means: Apparently all birds have the ability to do it. However, not all display that particular kind of unihemispheric sleep all the time. in case of the gull only 17% of the time it sleeps. The mallard duck would then be one of the few always displaying the USWS kind of sleep.

The answer appears in the excellent (though slightly dishonest, IMHO) movie Winged Migration. But I don’t want to spoil the puzzle for anyone else…

Thank you all for your answers! I’ll have to update the answer I had - it seems sleeping with the half the brain is quite common among birds.

Here’s the answer I had:

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.

Obvious from the posted links, but still worth mentioning i think:
a lot of sea mammals also display this behaviour.