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Nature’s Secrets

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Nature’s Secrets: How do birds rest during migration?

By Meg Lowman

If aviation engineers could apply the wisdom of the chimney swift, several troublesome problems of aeronautics could be solved. Pilots, for example, would never have to worry about the amount of gasoline in their tanks. The chimney swift refuels on the wing, spends almost its entire waking life in the air, and never, except by accident, touches the earth.

Rachel Carson

Every autumn, many millions of birds migrate from northern breeding grounds to equatorial locations. This annual flight is not only extraordinary in terms of time and energy but also raises questions about the physiological issue of sleep for birds. Some birds migrate long distances, and others only shift regionally. So how do birds rest during migration, and what are the consequences for sleep in migrating animals?

In 2011, the Swiss Ornithological Institute affixed electronic tags to the Alpine swift to monitor their movements. This bird spends the summer breeding in Europe, but winters in Africa, many thousands of miles away. Thanks to radio-tagging, scientists found that these birds were always in the air – and never alighted on land. The tags only recorded data every four minutes, so the birds could have landed intermittently. But this amazing discovery indicates that some birds almost never sleep in the conventional sense. In their aerial existence, swifts feed on insects in the air columns and do not need to be land-bound for feeding. However, they do need to breed, which undoubtedly requires some time on land, and the process remains a mystery. ...

Sleep is incredibly diverse across the animal kingdom, with some animals sleeping two hours, and others sleeping 20. Many factors influence sleep in wild animals, including food, predation and trophic level (position in a food chain). It’s generally thought that every species has a specific sleep “quota” – an average amount that they sleep every day. However, recent research has shown that there is some flexibility in sleep with some species. Just prior to migration, white-crowned sparrows reduce their sleep time by two-thirds yet do not show any of the cognitive impairment generally associated with sleep deprivation. This migratory restlessness has been observed in other bird species, and can be induced in the lab by artificially shortening the day length. Pectoral sandpipers, while breeding in Alaska during a very short summer, stay awake for almost two weeks to maximize breeding opportunities. The males that sleep the least sire the most offspring – a rare case where sleep deprivation is an evolutionary advantage.

Meg Lowman, Ph.D., a forest canopy expert, is senior scientist at the N.C. Museum of Natural Sciences and research professor at N.C. State University.

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