Small birds like swifts and swallows that migrate long distances have been predicted to travel up to 500 kilometres per day, but new evidence shows that one species of swift can cover far more ground than that.
Susanne Åkesson and Giuseppe Bianco at Lund University in Sweden have shown that common swifts (Apus apus) can fly an average of 570 kilometres per day. The fastest swifts covered 832 kilometres a day.
Swifts can fuel up on insects without landing, which allows them to remain in flight for about 10 months of the year.
“They are very special birds. Thanks to their small size and also this fly-and-forage strategy, they can generate very high migration speeds over time,” says Åkesson. “For ordinary birds, like waders and ducks and songbirds, they need to be spending some time sitting on the ground foraging before they fly, but the swifts, they can feed a little bit every day and feed on the wing.”
Åkesson and Bianco used geolocators to track the migration patterns of common swifts that breed in Sweden’s northern province of Lapland and migrate to areas south of the Sahara desert. The swifts leave Lapland each year around mid-August and return north at the end of May.
The pair tracked 19 swifts in the autumn, when the migration included 20 flying days and 22 stopover days on average, and 20 swifts in the spring, which involved 15 travel days with five stopover days, on average.
In the spring, “they have a chance to find very good tailwind conditions, especially at high altitudes”, says Åkesson. The ease of finding insects in the spring may also make it a shorter migration than the autumn trip.
The pair’s analysis of weather along the migration route showed that the birds seemed to time their departure by anticipating future wind conditions. Åkesson says it’s not clear how the swifts do this, but previous studies have shown that birds are sensitive to air pressure changes.
“With populations of migratory species declining worldwide, this knowledge can help identify the threatening processes that birds are exposed to during their migrations and the potential impacts of environmental change for populations,” says Christine Howard at Durham University in the UK.
While common swifts breed across Europe and Asia, the population Åkesson and Bianco studied were from breeding sites in the northernmost part of the species’ range in Europe. The northern swifts seem to be swifter than their slightly larger southern cousins but the reason for these differences in speed isn’t yet known. It may have to do with the routes taken and wind conditions at the time of departure, as the northern swifts migrate later in the season, says Åkesson.
Journal reference: iScience, DOI: 10.1016/j.isci.2021.102474
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