Research using miniature tracking devices suggests that swifts eat and sleep in the sky, as some birds did not land at all during their migratory period

Swifts already hold the title of the fastest fliers on Earth and now the small soot-brown birds have been revealed as one of nature’s greatest endurance athletes, after scientists discovered they spend ten months of the year entirely airborne.

Using miniature trackers, scientists observed that some birds did not land once during their migratory period, suggesting that they eat and sleep in the sky.

Anders Hedenström, an ecologist who led the work at Lund University in Sweden, said: “It’s amazing ... We knew they were extremely well-adapted to flight. They have very long and narrow wings and a streamlined body. They’re like Formula One cars or greyhounds.”

While some other species, including frigate birds and alpine swifts, are known to stay in flight for periods of months, no other bird has specialised in an aerial lifestyle to the same extreme as the swift.



Swifts were already known to feed on insects in flight and collect feathers floating on the breeze to build nests - now it appears they also nap on the wing.

“Since all organisms we know of need to sleep to some extent, we can only assume if they sleep at all it must be while they are airborne,” said Hedenström.

Ornithologists have long suggested that the birds must spend most of their time in the air, partly because no swift roosts have ever been found in Africa. The latest study, published in Current Biology, was the first to continually track the birds’ movements and confirm this.

Using miniature geolocators, which also monitored motion and altitude, the scientists followed 19 common swifts over a two-year period.

The birds were tracked as they migrated from Sweden, where they were tagged, to the Iberian peninsula down through West Africa to Central Africa. On their return the birds habitually travel via Liberia, where massive groups congregate for a week or two to eat termites. “Local ornithologists describe it as a real spectacle,” said Hedenström. “There must be several million swifts.”



There was some variation between the birds, with one settling in an upright position, possibly roosting, for four nights, and others stopping for a few hours at a time. However, some of the birds showed no sign of ever having landed during their ten-month migration. Overall, the birds spent more than 99% of their non-breeding period in flight.

The birds were seen to soar to heights of two to three kilometres at dawn and dusk and the scientists speculate that they might have “power naps” while gliding back down to lower altitudes.

“That’s just a guess,” said Hedenström. “From a human perspective it would be easier to take a nap when gliding when you wouldn’t be disturbed by flapping your wings.”

In the future, the team hopes to record the birds’ brain activity while they are in flight to test this theory.

Each year, common swifts spend around two months in Northern Europe, including the UK, where they breed, lay eggs and raise chicks, before migrating thousands of miles to Africa where they spend most of the year. Their streamlined arrowhead form makes them the fastest species in level flight - in a steep dive the peregrine is the fastest of all birds.

Ben Andrew, a wildlife adviser at the Royal Society for the Protection of Birds, said that young swifts may spend even longer periods airborne, because they do not return to Europe to breed for several years. “When young swifts are born in this country and leave their nest they don’t come back straight away,” he said. “Some of them could be in the air for four years before they land again.”

The common swift is on the RSPB’s amber list of species that are showing signs of decline, which Andrew said is linked to their nesting sites in roofs being closed off in modern buildings. “We’ve slowly but surely been blocking them out,” he said. “We’ve been working with builders to put in swift holes and swift bricks, which is helping.”