Common swifts enter hibernation-like torpor on cold nights

By Christa Lesté-Lasserre

A migratory bird that almost never stops flying sometimes slips into a brief, hibernation-like state inside its nest during chilly breeding periods.

When it is not breeding, the common swift (Apus apus), a small European and Arctic bird, spends more than 99 per cent of time in the air, and even flies while sleeping.

However, when cold weather hits the species’ breeding sites in Europe, the birds occasionally lie still in their nests for up to 22 hours in an energy-saving mode known as torpor, says Arndt Wellbrock at the University of Siegen, in Germany.

“Swifts are flying almost non-stop 10 months out of the year, even throughout the night,” he says. “So it’s a little bit strange to find the birds sometimes in these torpor states, very cooled down and non-active, when we know that, normally, they need very little time to rest.”

Swifts feed on flying insects, Wellbrock says. But during harsh weather, their prey become less active, and thus less available. By going into torpor – which is not the same as sleep – swifts temporarily drop their energy needs. This reduces brain and muscle activity, allowing them to survive longer without food.

To discover evidence of torpor in the birds, Wellbrock and his colleagues placed miniature temperature loggers in about 50 common swift nests, which the birds had built beneath a highway bridge in Germany. The loggers record the birds’ body temperatures.

Over eight annual breeding seasons, the team registered a total of 22,357 night-time temperature readings. Over two of the breeding seasons, the team could also calculate the metabolic rates of birds that took up residency in seven wooden nest boxes equipped with technologically for measuring oxygen and carbon dioxide levels.

The researchers found rare, but significant, temperature drops averaging 8.6 °C, bringing nest temperatures to an average low of 24.3 °C. These torpor states lasted 10.8 hours on average, with one bout lasting more than twice this length of time. They occurred on less than 6 per cent of the nights in an entire breeding season, says Wellbrock.

Swifts usually drop their metabolic rates by about 33 per cent at night compared to daytime, he says. But when they were in torpor, those rates dropped an average of 56 per cent below daytime rates. That means the birds were using about 70 per cent less oxygen – and consequently producing about 70 per cent less carbon dioxide – as they did on nights when they did not enter torpor.

The results provide clear evidence of torpor in these otherwise highly active, migratory birds, says Wellbrock. Surprisingly, they also suggest that the birds enter torpor together as a family of up to six birds. “We actually do not know how they coordinate this,” he says, adding that the question warrants further research.

The findings also might hint at swifts being part of a more evolutionarily primitive line of birds, Wellbrock says, as torpor is an ancestral trait in land animals.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2021.0675

Sign up to Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth’s other weird and wonderful inhabitants