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.
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. Some birds migrate long distances, while others only shift regionally. So how do birds rest during migration, and what are the consequences for migratory sleep deprivation?
In 2011, the Swiss Ornithological Institute affixed electronic sensors to alpine swifts to monitor their movements. This species spends summers breeding in Europe, but winters in Africa, many thousands of miles away. Thanks to electronic tagging, scientists found that these birds were always aloft in the winters, feeding in the air columns. The tags only recorded data every 4 minutes, so these birds could have landed intermittently. But these results indicate that the swifts go long periods without sleep in the conventional sense.
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 is generally thought that every species has a specific sleep “quota,” or an average amount that they sleep every day. However, recent research has shown that there is some flexibility in sleep requirements for 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 length of day. During the very short Alaskan summer, pectoral sandpipers stay awake for almost 2 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.