In September of 2011, three alpine swifts took to the air in southwest Africa, and stayed there for almost 200 days. They fed on the wing. They slept on the wing. By the time they firmly settled back on solid surfaces, it was April of 2012 and they had travelled across the Sahara to the Mediterranean.
By fitting the birds with tiny trackers, Felix Liechti from the Swiss Ornithological Institute showed that they probably flew non-stop for almost seven months. It’s possible that they landed occasionally, but very rarely and never for more than a few minutes at a time.
Birds are known for its mastery of the skies, but the swifts may be the most capable of them all. Their long wings make them fast and manoeuvrable, allowing them to scythe through the air in search of small insects and other “aerial plankton”.
Their skill is so great that many textbooks claim that common swifts can stay on the wing for their entire lives, landing only to breed and incubate their chicks. (The group’s name, Apodidae, comes for the Greek words for “without feet”.) But this claim is based largely on European radar-tracking studies, which showed that the birds often fly throughout the night—hardly convincing evidence. “No one had ever proven it by following individual birds,” says Liechti.
Liechti changed that by catching six alpine swifts at a breeding colony in Baden, Switzerland, and fitting them with light data-loggers. He released them, and they flew to Africa where they spent the winter. The next spring, they came home after a brief stop-over in Northern Africa and Southern Spain. Liechti recaptured three of them.
The data-loggers recorded the birds’ activity and the angle of their bodies (their pitch) every four minutes, along with the light levels around them. Liechti used the data to reconstruct what the swifts did over the previous seven months.
The combination of activity and pitch was very important. An active swift is almost certainly flying, but a motionless one could either be resting or gliding. It’s their pitch that gives the game away—it’s very stable if they’re flying, but highly variable if they’re resting.
The recordings showed that while breeding in Switzerland, the swifts fly during the day and roost at night, probably hanging from trees or cliffs. The same happens while they migrate, although they also clock in a lot more night-time flying.
But when they reached their wintering grounds in Africa, their behaviour changed. The data-loggers showed that the swifts’ day and night movements were almost identical, and they never showed the distinctive pitch or a hanging or roosting individual. They were either flying non-stop or coming very close to it.
Liechti was surprised. Alpine swifts are big birds. They’re twice the size of common swifts, and have wingspans of up to 22 inches (57 centimetres). “We didn’t expect them keep flying all day and all night long during their stay in Africa,” he says.
The swifts must be able to do all of their normal activities, including sleeping, while in the air. How do they manage? “I don’t know!” says Liechti, laughing. “We can only assume that they do something similar to dolphins.” Dolphins can send one half of their brains to sleep at a time, allowing them to stay alert after days of constant vigilance. It’s unclear if swifts and other birds can do the same, but Liechti would be impressed if they can. A dolphin can hang in the water without any problems, but flying requires continual effort, small adjustments, and a lot of energy. It’s a harder act to perform constantly.
And why do they fly continuously? Again, Liechti has speculations rather than answers. They may exploit food sources that other birds can’t touch, avoid predators by flying through the night, or stay beyond the reach of parasites like malarial mosquitoes. “These aren’t very convincing,” he admits, “but for sure, there’s a cost to staying in the air, so there must be a benefit.”
A better question might be: since they’re capable of staying airborne for so long, why do they ever land? It may have something to do with their food. In northern latitudes, they may be forced to land because there’s not enough food in the air to sustain their long non-stop flights. They may be the world’s greatest aeronauts, but they’re still dependent upon lesser ones.
PS: I’m pretty sure I remember hearing in a wildlife documentary that the Arctic tern flies non-stop on its way from the Arctic to Antarctica and back again. Liechti tells me that this isn’t true—these champion migrants do settle on the water on their travels to feed and rest.
Reference: Liechti, Witvliet, Weber & Bachler. 2013. First evidence of a 200-day non-stop flight in a bird. Nature Communications. http://dx.doi.org/10.1038/ncomms3554