The common swift is “the greyhound of the sky,” Anders Hedenstr?m said. Shaped like a torpedo, with long, blade-like wings, it effortlessly swoops and soars through the air, riding the wind like a creature out of mythology. For millennia, bird lovers have admired its aerial agility and seemingly endless flight.
But even the most ardent ornithologists were amazed to learn that common swifts are capable of flying for 10 months without once touching land — a world record for time on the wing.
“It’s the most extreme example of nonstop flight … that we know of,” said Hedenstr?m, a biologist at Lund University in Sweden and lead author of a study published Thursday in the journal Current Biology examining common swifts’ astonishing abilities.
Hedenstr?m and his colleagues spent two years tracking 13 swifts via tiny light sensors and accelerometers loaded into backpacks they attached to the birds. The light sensors allowed them to geolocate the birds by tracking the time of sunrise and sunset. The accelerometers tracked the beating of the swift’s wings and the speed of their forward movement, so scientists could calculate how far and fast the birds were traveling and the amount of time they spent on the ground.
The data revealed that common swifts — which make a 10-month journey from Northern Europe to Central Africa and back each year — spend 99.5 percent of their migration in the air. When they did touch down, on a tree branch or patch of ground, it was only for an hour or two. Then back into the air to continue their marathon journey.
“It’s absolutely minuscule, the time they actually spend resting,” Hedenstr?m said.
Three of the birds never landed at all. Instead they spent their entire migration aloft, traveling more than 10,000 miles without rest. No other migratory bird — not even the frigate bird, which spends weeks on the wing during long ocean crossings — is known to spend so long in the sky.
Common swifts must do everything they need to survive while aloft. They mate in the air and consume airborne insects. They drink by gliding over water and dipping their beak in to take sips. It’s assumed that they even sleep while in flight — Hedenstr?m’s study found that the birds ride updrafts to extremely high altitudes at dawn and dusk, and it’s possible that they nap while drifting downward.
The birds are members of the ancient bird lineage Apodidae, which may extend as far back as the Cretaceous Period more than 65 million years ago, and they are perfectly evolved for flight. Their long, narrow wings are adapted for maneuverability and for soaring long distances on currents and a single flap. Their crescent-shaped tails can be manipulated for extra efficiency.
But they fare poorly on land. The birds’ legs are so short that ancient observers thought they didn’t exist at all. Young swifts are unable to take off from flat ground — they have to drop out of trees and then soar upward. Hedenstr?m believes that this may explain their penchant for staying airborne: The birds are simply more at ease in the sky. Not to mention safer from predators and other threats.
Swifts’ ability to fly for months at a time remains a biological enigma.
“It’s going to be fascinating to learn more,” Hedenstr?m said.
