How and where do birds sleep?

(Cross-posted from Backyard Birder.)

There's a really interesting piece in Slate.com today about the way that birds sleep. It seems that a lot of people have misconceptions about that. They think that birds sleep in their nests. But the sole purpose of nests is to provide a place to hold eggs and chicks, and once that purpose is fulfilled, that nest is so nasty that no self-respecting bird is going to want to sleep in it.

The main concern of a bird looking for a bed for the night is to find a place that is safe from predators. A second concern may be to have some protection from the weather, but first and foremost is always the fear of predators.

The predator problem is such a threat to birds' survival that they have evolved a brain that will help them to meet it. They are essentially able to sleep while one-half of their brain stays awake and alert for danger.

This technique, called unihemispheric slow-wave sleep (USWS) allows the bird to have one hemisphere of their brain in a deep sleep while leaving the other hemisphere awake and alert. They are also able to turn USWS on and off depending on how safe their roost is. The Slate story gives the example of a large flock of ducks roosting on an open lake. The birds at the center of the flock are pretty safe and can shut down completely, while the birds on the outer edge are more vulnerable and have to keep half the brain alert.

So, birds, it turns out, sleep - it's called "roosting" as anyone who has ever raised chickens knows - in a lot of different places depending on what kind of birds they are. Big water birds like geese and ducks will generally sleep floating on the water. Big wading birds, like herons and egrets, usually will sleep perched in waterside trees. Shorebirds sleep in large flocks and utilize USWS and they are cryptically colored to fool predators. Hawks, eagles, and owls sleep pretty much wherever they want to. Grouse and quail which are favorite foods of so many predators depend on their coloring and on vegetation to hide, and they also generally sleep in flocks with half a brain awake. Woodpeckers roost in protected places like tree cavities or under roofs and bridges. Birds like crows, swallows, swifts, and starlings roost in large - sometimes gigantic - flocks for safety.

But the birds that we are most familiar with, the perching birds from the order Passeriformes, tend to roost in dense vegetation. These are our backyard birds like cardinals, sparrows, jays, and finches and around dusk you can see them flying into bushes, hedges, and trees where they will grab onto a a twig that suits them and settle down for the night.

So the next question is, how do they keep from falling off their perch while they are asleep? The answer once again is evolution. The perching birds have evolved "flexor tendons" in their legs that involuntarily clasp shut when a bird sits on a perch and they won't relax and release again until the bird straightens its leg. The grip is so tight that some birds, such as hummingbirds, have been observed sleeping upside down in apparent safety and comfort.

So the next time you see a cartoon of a little bird crawling into its nest at night and pulling a tiny blanket up to its beak, you'll know that it is just that - a cartoon. The true story of how birds sleep is a lot more complicated and a lot more interesting.