Common swifts (Apus apus) remain airborne for 10 months of their non-breeding period, according to a new study by Lund University ornithologists.

While there had been examples of birds remaining in flight for periods of months, including frigate birds and alpine swifts, the evidence on common swifts sets a new record.

“This discovery significantly pushes the boundaries for what we know about animal physiology. A 10-month flight phase is the longest we know of any bird species — it’s a record,” said lead author Prof. Anders Hedenström, from the Department of Biology at Lund University, Sweden.

“When the common swifts leave their breeding site in August for a migration to the Central African rainforests via West Africa, they never touch ground until they return for the next breeding season 10 months later,” he explained.

“Some individuals may roost for brief periods, or even entire nights in mid-winter, but others literally never landed during this period.”

“The birds likely save energy during the day by gliding in upward currents of warm air. But they also ascend to high altitudes each day at dawn and dusk.”

Prof. Hedenström and his colleagues at Lund University followed 13 individual birds, some of them for two years in a row.

Using a new type of microdata log, attached to each bird, the researchers were able to determine whether the birds were in the air or not, their acceleration, and where they had been at any given time.

The results show that some of the birds landed during short periods at night, sometimes during an entire night.

But even these birds spent more than 99.5% of their 10-month migration and hibernation period in the air.

Data from other birds show that they did not land a single time in 10 months.

The birds which had never landed had all molted and gained new flight feathers (wing and tail), while the majority of those who, on some occasion, landed had not molted their wing feathers.

“Whether they molt or not could indicate small differences in their general condition or burden of parasites, and explain the flight behavior of individual birds within the species,” Prof. Hedenström said.

The new knowledge about the swifts has already generated new questions such as how they handle the high energy consumption during 10 months in the air, and how they fly and sleep at the same time?

“They might do as the frigate bird and sleep while gliding,” Prof. Hedenström said.

“Every day, at dusk and dawn, the common swift rises up to an altitude of about 6,500-9,800 feet (2-3 km). Perhaps they sleep during a declining glide, but we’re not sure.”

“Despite the high energetic costs associated with all that flight, common swifts also manage to live surprisingly long lives, contrary to popular notions about living hard and dying young. There are documented cases of common swifts living to the age of 20.”

“In that time, the accumulated flight distance equals seven round-trip journeys to the Moon. And that means there are many more intriguing questions to ask and answer about the birds’ physiology,” Prof. Hedenström added.

The team’s findings were published online Oct. 27 in the journal Current Biology.

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Anders Hedenström et al. Annual 10-Month Aerial Life Phase in the Common Swift Apus apus. Current Biology, published online October 27, 2016; doi: 10.1016/j.cub.2016.09.014