Another Life: Aerial mating and breast to breast battles

I miss the coming of city swifts – the change in the sky from one day to the next. In my rooftop bachelor pad in Ballsbridge, circa 1965, I enjoyed the sudden swirl of fresh life above the chimneys, the space-carving arcs of flight, each with its gliding cursor of dark wings. Occasionally, when the traffic eased, I could even hear them calling – screaming, if you like.

Most Dubliners, heads bent as they hurry or cocked to a mobile, may take longer to register the arrival, in the next few days, of the last spring migrants from Africa. In the Georgian, horse-drawn city, swifts could scarcely be ignored as they swooped down and dashed through the streets to feast on a haze of golden dung flies, sometimes knocking off hats as they flew.

They were nesting, two or three pairs at a time, in crevices beneath the window sills of red-brick houses, also in their eaves and soffit boards, and in the more natural crannies of cliffs at Howth and Bray Head. In today's cities of concrete and glass there are few such accommodating apertures, and a European effort has sprung up to check the global decline of Apus apus, at least as an urban visitor.

Amateur enthusiasts
In Ireland this has been led by amateur enthusiasts in the North (saveourswifts. org.uk) who have established swift nesting colonies on their houses. Helped by ornithologists in the Republic, they promote the use of "swift bricks" – hollow bricks made of woodcrete. Like garden tit boxes or bat boxes, these provide new breeding posts at a growing number of modern rooflines around the country.

Some are fitted to plant rooms on top of Dublin's civic offices (viewable from Ormond Quay). To quote from last month's international conference on swifts in Cambridge in England, "Screaming swifts are the sound of summer from Dublin to Beijing." But they are by no means a pleasure exclusive to big cities, and the conference welcomed Lynda Huxley of Galway-Mayo Institute of Technology in Castlebar.

She heads the college’s “green campus” committee, which, two years ago, installed a dozen nest boxes under the eaves of the refurbished old mental hospital. Last year swifts were filmed exploring the boxes and even bringing nesting material into them. This year infrared cameras are installed in all of them, and events in the four most active will be streamed live on the institute’s home page (gmit.ie) – the only such peep show in Ireland.

It's 70-odd years since the British ornithologist David Lack persuaded swifts to exchange nesting space on ledges high in a tower of Oxford University Museum of Natural History for a set of nesting boxes built in the same space. As the birds settled to their annual visits, the backs of the boxes were replaced by glass. Summer after summer, Lack and his wife perched under the tiles, in the dark, to watch – and even, lifting the lid, to take out eggs and young for weighing and measuring.

The birds seemed not to mind this intrusion from the rear, while fighting bitterly any incoming swift attempting dispossession. They could be locked in violent breast-to-breast battle in the close space for hours – “it looks horrid,” wrote Lack – so the Castlebar video may occasionally be well up to television drama. And the nestlings, too, were “hideous” – quite naked and pot-bellied. But, as Lack said, “this distorted monster is really a miracle of adaptation”, storing fat for its eventual, unaided launch into a most uncertain air.

Aerial mating
The consensus that swifts spend nine months of their year in the sky, touching down only to breed their young in a hole, is comparatively new. It's true that Gilbert White of Selborne, in the 1790s, accurately described their aerial mating. ("Both of them sink down together for many fathoms with a loud piercing shriek.") And many modern scientists were long convinced that swifts also did their sleeping on the wing.

But it was the mid 20th century before respected observers felt like stating it as fact. David Lack, watching swifts rising at dusk and disappearing out over the sea, decided “the mystery of where migrating swifts spend the night has been solved. They go up into the sky.” Speculating on their “catnaps” to avoid bumping into each other, he anticipated research on the widespread avian ability to sleep with one eye and half the brain open.

The idea that swifts ascend specifically to sleep may, however, need modification. Dutch scientists, using weather radar to track swift altitudes, found them ascending up to 2,500m at twilight, but doing it again at dawn. Among the reasons, they thought, could be to get a better view of where they were going, and of the kind of weather ahead. One presumes that they come down in between.