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This great reed warbler can migrate above 6000 meters.

Tiny songbirds cross deserts and seas by soaring three times higher than usual

Migration ecologist Sissel Sjöberg had long wondered how tiny birds like the great reed warbler can make it across long expanses of water or desert on their epic migrations. Though just half the weight of a golf ball, they fly 7000 kilometers between Northern Europe and sub-Saharan Africa twice a year. Now, a new study may have the answer: These nighttime flyers soar well into the day—and at heights of up to 6000 meters, three times as high as they normally fly.

“That’s totally unexpected,” says Martin Wikelski, an ecologist at the Max Planck Institute of Animal Behavior who was not involved with the work. “It’s like you are building mud houses and suddenly somebody [else] builds a skyscraper. And all you can say is, you didn’t know you could do that.”

Birds from more than 4000 species, including thousands of songbirds, migrate across long distances each year. Although researchers have been able to track the migration paths of larger birds such as geese, it’s challenging to outfit small songbirds with the tracking devices that make such research possible, says Felix Liechti, an ornithologist at the Swiss Ornithological Institute who pioneered such tracking technology, but was not involved with the new research.

To see whether she could change that, Sjöberg joined the lab of Lund University ecoimmunologist Dennis Hasselquist. The pair turned to great reed warblers (Acrocephalus arundinaceus), relatively large songbirds that summer at a lake near Lund University. Scientists there have monitored the species’ breeding and behavior for 40 years, even developing custom data loggers for the birds.

To track the warblers—which are about half the size of an American robin—Sjöberg, Hasselquist, and colleagues outfitted them with the data loggers—small backpacks that can monitor when, how high, and where the warblers fly on their semiannual journeys between Sweden and Africa. (That feat was only possible, Sjöberg says, because the warblers can grow up to three times bigger than the average songbird.) The researchers put the backpacks on 63 birds and retrieved useful data from 14.

As expected, the migrants usually spent their nights flying and rested during the day. But most birds that hit a water or desert crossing near dawn kept going as the Sun rose, climbing high into the sky. One bird was airborne for more than 32 hours. And instead of staying at typical altitudes of less than 2000 meters, some birds soared higher than 6000 meters, Sjöberg and colleagues report today in Science. That’s not quite as high as the 8000 meters bar-headed geese hit as they skim the Himalayas, but it’s still a tall order, Sjöberg says. “We have never even imagined that songbirds regularly would fly this high.”

Such altitudes can be stressful. At 6000 meters, oxygen is scarce and the temperature is below freezing. Somehow the birds are able to cope. “The ease at which they [fly that high] is amazing,” Wikelski says. Like other migrating songbirds, warblers have relatively large hearts and air sacs in the lungs designed to increase the rate and efficiency of oxygen exchange. Hard-working flight muscles likely keep the bird warm, despite the 22°C drop in temperature, Sjöberg says.

That cooling effect may be one reason the warblers fly so high, Sjoberg suggests. As the day dawns, the Sun’s rays can take a toll, and “the only way they can counterbalance this external heating is by rising steeply at sunrise,” especially when crossing the Sahara, she proposes. This need to stay cool may help explain why other birds often migrate at night, adds Melissa Bowlin, an ecophysiologist at the University of Michigan, Dearborn. She expects other songbirds also soar to such great heights during the day, making the work ”important.”

Liechti says he is excited about the results. His work using radar data to monitor animals moving across the western Sahara had suggested some birds fly during the day, possibly taking advantage of favorable tailwinds. But, “We just tracked unknown blips … [these other researchers have] tracked single known individuals along the whole flyway.”

The results, he adds, “show there are still a lot of unexpected flexible behaviors of animals to be discovered.” Sjöberg agrees: “It is important to know how flexible birds [can be] as it tells [us] about the possibilities birds may have to adapt to” climate change.