Common swifts have one of the longest migrations in the world, travelling some 14,000 miles every year from the UK to spend their winter in Africa. Whilst this in itself is astounding, new research from the University of Lund has now revealed that incredibly, they spend a whole 10 months in the air without landing. This poses new questions; how do they maintain their energy during this time? How do they sleep?
By attaching microdata loggers to 13 birds, the researchers were able to track their movements over a long period of time. For some birds this was up to two years of travel, during which the loggers sent back data about location, acceleration, and whether they were airborne or not. The results, published in the journal Current Biology, found that some birds were able to continue flying for 10 months, while those that landed for short periods still spent 99.5 per cent of their ten-month migration in the air.
“This discovery significantly pushes the boundaries for what we know about animal physiology,” says Professor Anders Hedenström from the Department of Biology at Lund. “A ten-month flight phase is the longest we know of any bird species – it’s a record.”
Interestingly, the birds that never landed moulted and gained new flight feathers in the wings and tail, but those that landed didn’t. Researchers speculate that this could indicate small differences in body condition, or burden of parasites. Furthermore, the research also showed that the birds’ flight activity was lower during the day than at night, likely a result of the birds saving energy gliding on warm, upward air currents during the day, explains Hedenström.
Sleeping on the job
However, the researchers remain unsure on how the birds sleep during this time.
“They might do as the frigate bird and sleep while gliding. Every day, at dusk and dawn, the common swift rises up to an altitude of about two-three kilometres. Perhaps they sleep during a declining glide, but we’re not sure.”
Given the accumulated flight distance is equal to that of seven round-trips to the Moon, and swifts have been known to live up 20 years old, it is unsurprising Hedenström and his colleagues hope to explore this field in future research, with many more fascinating questions to ask and answer about the birds’ physiology.