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Each spring, crowds of warblers make a perilous journey of several weeks between their winter homes in Africa and their summer breeding grounds in Europe.
The small brown and white songbirds travel thousands of kilometers across the Sahara and the Mediterranean Sea to their destination. It is an exhausting and arduous journey, and the warblers make numerous stops in the pits to rest and refuel en route. During these stopovers, birds must catch up on sleep, replenish their fat reserves and somehow avoid being eaten by predators, including hungry raptors migrating alongside them.
A study published Monday in Current Biology revealed a way in which migrating warblers manage these dangers and requirements: they adjust their sleep posture according to their physical state and their physiological needs. Plump, well-muscled birds tend to sleep with their heads upright, while leaner warblers bend into the head, a posture that makes them more vulnerable to predation while helping them retain the energy they need. are in need.
"Migratory warblers need to compromise between staying safe and saving energy," said Leonida Fusani, behavior physiologist at the University of Vienna and lead author of the paper.
Dr. Fusani worked with a PhD student, Andrea Ferretti and several other colleagues studied the garden warbler that had stopped on the island of Ponza during their spring migration. The small, rugged island, off the west coast of Italy, is a popular stopover for northern birds, which usually arrive drained after a flight of over 300 miles over open waters.
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Researchers captured warblers with nets, then gave everyone a brief physical examination before transferring it to a custom cage for observation. Some warblers were thriving, with heavy bodies, large muscles and a lot of body fat. Others seemed to struggle, looking meager and exhausted by their travels.
Scientists found that birds in good condition slept more during the day than those in poor condition, perhaps because they did not need to spend so much time looking for food. And at night, the robust birds usually slept facing the front, head straight. The leaner birds, on the other hand, literally hid at night, turning their heads and placing them under the feathers of their shoulders.
Dr. Fusani, Dr. Ferretti, and their colleagues conducted several follow-up studies to try to understand this pattern. In one of them, they used a thermal imaging camera to monitor the body temperature of the sleeping warblers in the intact position. These birds have lost heat mainly through the head, especially from the area around their eyes. By placing their head in their feathers, lean warblers could minimize this heat loss, the researchers said.
In addition, when the scientists placed the birds in a chamber of respiration during the night, they found that the dormant warblers had lower metabolic rates than those who slept without being bent.
Together, the results suggest that migrating birds that arrive in poor condition at their stopover can adapt by choosing a sleep posture that preserves their steadily dwindling energy supply.
"Evolution has created these behavioral flexibilities that allow birds to compensate if they have to use more fat reserves on that leg because they have had a storm or the temperature has become colder than expected," m said Scott McWilliams, who studies avian ecology and physiology at the University of Rhode Island, is the author of the article.
But if the hidden position has such advantages, why do not all the warblers use it? Dr. Fusani and his colleagues hypothesized that posture could make birds less alert to potential threats. To test this theory, they simulated the sound of a predator by approaching a record of crisp leaves during the sleep of warblers. Indeed, it took longer for the birds in the hidden position to react faster than those who slept their heads forward. "The cost of hidden sleep is a slower reaction time," Ferretti said.
The research could help shed new light on the benefits and overall function of sleep, the scientists explained, as well as how migratory animals can exploit it to increase their chances of survival.
"We never suspected that sleep patterns were an important factor in the migration strategy," said Dr. Fusani.
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