Every July, young common swifts leave their European roosts and migrate to western and central Africa. They’ll only be back in the following June, and they’ll spend the intervening 10 months almost continuously in the air. They might travel to Africa, but their feet never meet African soil.
“They feed in the air, they mate in the air, they get nest material in the air,” says Susanne Åkesson from Lund University in Sweden. “They can land on nest boxes, branches, or houses, but they can’t really land on the ground.” That's because their wings are too long and their legs are too short to take off from a flat surface.
As a result, common swifts are among nature’s greatest aeronauts, superbly adapted for a life spent largely in the skies.
People have long suspected that swifts stay airborne for long stretches of time, but no one could confirm it. Åkesson and her husband, Anders Hedenström, both from Lund University, did so by fitting 19 of the birds with lightweight data loggers in 2013, and recapturing them one or two years later. The loggers had regularly recorded local light levels, which the duo could use to pinpoint the birds’ global position. And they had recorded the swifts’ acceleration, activity, and body positions, which revealed if they were flying or resting.
The data revealed that some swifts were almost never inactive during their long migrations. For example, between September 2013 and April 2014, one of the birds rested for just four nights in February. The following year, it never rested for a full night; over the same period, it stopped for just two hours.
A few other swifts did rest more often. Åkesson and Hedenström suspect that this was because they hadn’t finished replacing their wing and tail feathers during their annual molt. The old feathers might have compromised their ability to stay permanently aloft, or their inability to molt fully might reflect some underlying health problem that affected their flight. Either way, it’s not like they were actually grounded. If they rested, it was just for a few hours. At most, they spent just 0.64 percent of their migration on terra firma.
Other scientists have found similar results. In 2013, Felix Liechti from the Swiss Ornithological Institute used similar data loggers to show that three alpine swifts probably flew nonstop for 200 days, on their long migration from Switzerland to Africa and back again. That was especially surprising because alpine swifts are twice the size of common swifts, with wingspans of up to 22 inches.
Such long nonstop flights imply that swifts must be able to sleep while flying. This August, Niels Rattenborg of the Max Planck Institute for Ornithology in Germany proved that frigate birds—large seabirds with piratical tendencies—do the same. He fitted brainwave sensors to 15 frigatebirds and showed that they sleep mid-flight, for about 40 minutes a day and just a few seconds at a time.
There’s no brain sensor small enough to record their mental activity of a flying swift, but they surely must be taking midair naps too. Maybe they do what dolphins do, sending one half of their brains to sleep at a time so they can stay continuously alert.
Why bother? Why should a bird evolve to be continuously airborne, rather than to land occasionally? “When you’re airborne, you’re extremely mobile, and you can explore a dynamic food environment,” says Åkesson. Swifts eat insects, which might concentrate in dense swarms and move from one place to the next. By patrolling large swathes of sky, and never wasting time on the ground, swifts might be able to snap up more insect prey.