Another Life: The remarkable swallow and its curious behaviour

Michael Viney: They stay airborne for 10 months of the year, feeding and sleeping on the wing

The bluer the sky above our hillside, the emptier it seems to be. The swallows have given us a miss this year, the ravens have stopped travelling to carrion on the shore and we’re not in the right place for skylarks. The sky must still be full of invisibly drifting aphids, spiders and their like, but we also happen to be in the wrong place for swifts.

Go to some of Mayo’s country towns, however, and the sky is scythed by the scimitar wings of Apus apus and sometimes echoes to their screams. I remember, in my rooftop bachelor pad in Ballsbridge half a century ago, marvelling at the noisy aeronautics outside my window.

Mayo, indeed, is doing its best for a bird so threatened by homelessness, its numbers down by 40 per cent since 2008. But the tall, well-weathered architecture of the towns can offer high holes and ledges for “natural” nests , like the 17 in Ballina’s old Garda station and 14 at Ballinrobe’s old courthouse.

They’ve been counted by Swift Conservation Ireland, for whom Lynda Huxley has been leading a campaign. There are dozens more now, under the eaves of Castlebar’s old mental hospital, at the town’s swimming pool, and on the campus of what is now happily the Atlantic Technological University, or ATU.

Some of the boxes at the ATU are on camera, with views of swifts feeding their chicks. It may be as well not to see the chicks too early on, naked and pot-bellied — “hideous”, indeed, in the judgment of the famous David Lack.

He installed the first boxes for swifts high under the roof tiles of the Oxford University Museum in the 1950s, where his uncomfortable vigils recorded much of what we know about swifts breeding. It’s all in his book Swifts in a Tower, recently republished and edited by his son.

There are two remarkable conditions in the lives of swifts. One is that they can only launch into flight by diving off something high (their small, weak legs aren’t made for a ground launch). The other condition, now well confirmed, is that they stay airborne for 10 months of the year, feeding and sleeping on the wing. If, at night in very bad weather, they are rarely forced to pause, they may rest by hanging vertically on a cliff or a tree.

This and other extraordinary behaviour have been a challenge to ornithology, but the development of miniature technology is amassing a wealth of new research. Among its gadgets are micro data loggers, accelerometers, light-level geolocators, EEG recorders and rotational stereo videography.

The last, using a telephoto camera and a set of mirrors, has been used to track flying swifts as they forage for aerial insects to feed their young. It found , incidentally, that the swifts rarely performed high-acceleration turns. depending on a fast approach, rather than a sharp turn, to catch evasive prey.

From Germany, evidence from featherweight EEG brain recordings is offered in support of the common belief that swifts sleep on the wing.

Birds and mammals share two types of slumber — slow-wave sleep and rapid-eye-movement (REM) sleep. In the first, using one or both halves of the brain, the eye connected to the “awake hemisphere” stays open. This may allow the swift to navigate visually. But REM sleep needs both halves of the brain and usually reduces muscle tone, incompatible with flight.

Months in the air

Without its REM dreams, the paper by Niels Rattenborg suggests, the swift may need to land for “post-flight recovery sleep”. This, however, conflicts with the consensus that swifts spend 10 months in the air.

Another mystery of swifts is their gathering in flocks for screaming parties at sunset, followed by an ascent to high altitudes for a night of sleep on the wing.

A recent study using tracking radar was led by ornithologist Cecilia Nilsson at a colony of breeding swifts in southern Sweden. She remarks on the evening flocking of swifts, and their use of “very rapid vertical speeds” at both the ascent after sunset, up to 3,000m, and their descent in the morning.

Their purpose, using so much energy, has prompted many theories. Some suggest the birds may want to access star patterns or polarised light, or to see surrounding weather or distant horizontal landmarks. Dr Nilsson is sceptical of the need to fly so high and puts more social purpose on the flocking.

Meanwhile, at ATU Castlebar, Polish student Jaroslaw Majkusiak is gathering some 20,000 hours of continuous recordings from the 18 college nest boxes for analysis in a master’s degree. It will show how Mayo’s summer weather, from May to September, affects the birds’ breeding success. A basic key is the number of feeding visits the parents make to the chicks.