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One eye open. Migrating birds, such as this Swainson's thrush, watch for predators and catch up on sleep at the same time.

José Loaiza

Bird Brains Split Lookout Duty

What good is half a brain? Good enough for migratory birds to avoid predators when napping in the daytime. A new study finds that migrating birds take mini-naps during the day but only rest half their brains at a time, allowing them to keep one eye open.

Many migrating birds fly at night-- often, all night--to cut the risk of being seen by predators and to avoid overheating under the hot sun. Some of these flights are long, especially when crossing ecological barriers such as the Gulf of Mexico, says Frank Moore, a biologist at the University of Southern Mississippi, Hattiesburg. Sometimes, birds fly continuously for 24 hours. "That necessarily means they get less sleep when migrating," he says.

Weary birds face risks when they stop to rest along the way. Similar to people, sleep loss makes birds less vigilant, which leaves them vulnerable to predators. How birds manage to fly all night and not get eaten when they stop for the day hasn't received much attention from scientists, Moore says. "The problem just hadn't occurred to most people."

Moore and colleagues decided to answer the question by putting seven captive Swainson's thrushes, which are normally active in the day but fly at night during migration, into a cage and providing artificial sunrises and sunsets to mimic the migratory season. The scientists also implanted electrodes to monitor the birds' brain activity.

Occasionally, the birds would droop and seem drowsy for a few seconds but then kick back into wakefulness. During these mini-naps, they'd often close only one of their eyes. When researchers looked at their brain activity during these times, they saw that one hemisphere of the brain had electrical patterns resembling nighttime sleep, whereas patterns from the other hemisphere indicated wakefulness. The researchers, who published their findings online this week in Biology Letters, concluded that the birds were resting half of their brains at a time in order to catch up on sleep while staying on guard.

The findings make sense to Doug Levey, an ecologist at the University of Florida, Gainesville. "If you're a bird and you're mentally not very sharp, there will be serious consequences," he says. Half-brain sleeping has also been documented in marine mammals such as dolphins and whales, he says, so it could be a convergent solution to a similar problem. Levey's not convinced that the mini-naps compensate for lost sleep, though. "There are other explanations that could account for [the behavior]," he says, such as the birds' conserving energy by resting half their brains.

Charles Walcott, a neurobiologist at Cornell University, says he's "always a little suspicious" of electrical brain activity studies, because it can be difficult to tease out what's sleep and what isn't. He'd like to see the researchers use stricter quantitative standards to decide if an electrical signal represents sleep.