Every May my uncle notes down the first arrival of the swifts on his particular beat in north London. The birds are not quite as regular as the St George’s mushrooms which appear on precisely the same day – April 23 – every English spring, but the birds come some time between the 9th and 13th of the month. I haven’t asked him about this but I assume he finds it comforting, a sign in uncertain times that, as Ted Hughes put it, not entirely reassuringly, “the planet’s still working”.

The other day I was pleased to see a squadron of the dark hirundines with their scimitar wings flying over the little park near my house, preceded by their high-pitched, strangely stirring cries. But I’ve had the feeling that such swift-sightings, once commonplace, are becoming rarer. Then I chanced on a report by the Royal Society for the Protection of Birds pointing to a 40 per cent drop in numbers of our most aerial bird since the 1990s. Only 36,000 pairs of swifts now visit the UK; their summer stay always seemed intense and short – they will be off again, heading for sub-Saharan Africa, by the middle of August – but now it feels even more poignant. The RSPB warns that they may disappear for good from our skies by the middle of the century.

What is causing this steep decline? You would expect it has something to do with climate change, and it most likely does; spring is arriving earlier but the swifts haven’t cottoned on to that; droughts and extreme weather are affecting Africa, making it more difficult for the swifts to cross the great expanses of continent (they cover 20,000km between spring and autumn).

But there is another, maybe more important reason, which is also to do with us and the way we live, but much closer to home and much more within our control. Swifts, which seem among the wildest and least domesticated of all birds, spending nearly all their time on the wing (they even sleep and mate while flying), nest in buildings. Unlike swallows, which prefer barns and other country outbuildings, swifts like to nest in the eaves of town and city dwellings, warehouses, factories and offices. But recent trends in building both offices and houses have eliminated the nooks and crannies necessary for swift nests. The charity Swift Conservation (doesn’t it give you heart that such an organisation exists?) provides fascinating statistics for swift-friendliness of construction: the birds can nest in 10 per cent of pre-1919 buildings but only 1.4 per cent of post-1944 edifices. For anything built since 2000 the figure is close to zero.

Mostly unconsciously, we have been shutting the swifts out of our lives. Now the imperative to insulate our houses and make them airtight, to save on heating and therefore CO2 emissions, has made matters still worse.

But this is one of those relatively rare environmental problems which has an easy solution. The answer is swift bricks – concrete or terracotta bricks with hollow interiors and a small hole for swift access.

Swift Conservation recommends that some of these should be included in the highest row of bricks in a building, under the roof, at least 5 metres up and away from direct sunlight. And the fine new public leisure centre at Swiss Cottage in north London, built by Camden Council and designed by Terry Farrell, where I sometimes go to swim, has installed swift bricks, though all you can see from the outside is the tiny holes.

Why does all this matter? Why should we welcome swifts into our urban lives? One answer is the scientific one. Swifts are an example of biodiversity, part of the infinitely rich and complex interrelationship of species which makes up our world, and makes it liveable, for ourselves as well as other species. Swifts are great insectivores, eating up to 40,000 insects a day. We may not know exactly what effect their disappearance would have on the web of life, but for a long time now we have known that havoc can be caused when one thread of the great tapestry is broken.

“Untune that string”, as Ulysses puts it in Shakespeare’s Troilus and Cressida, “and hark what discord follows!”

But the scientific perspective, though crucially important, is not the only one; the American poet CK Williams sees through a different lens. Williams’s poem “Swifts” is not a lament or an ecological warning but a paean – a hymn of praise and love. His poem is a product of emotion, of a kind of sobbing exultation as he observes the swifts’ aerobatics on a summer evening. He feels he could stay and watch them for ever as they “bank, swoop, swerve, soar/ make folds and pleats in evening’s velvet”.

“Swifts” comes from a volume called Repair, much concerned with mending, both of mundane things and of the human heart. As they veer through the last light of evening, the swifts seem to repair the day’s losses, to make it good, as it was in the beginning.

harry.eyres@ft.com
More columns at www.ft.com/eyres