Fleeting visitors: Swifts arrive here in May and make their long journey back to Africa in August

Five swifts (apus apus) were high in the air over the rootops of the old red-bricked township of Rathmines last Sunday evening.

No creatures, except angels, are so wholly native to the upper air, according to the poet John Heath-Stubbs. The black screamers, rushing at evening above cities and borne up by rising thermals, are kin to the tropical humming-birds which can fly backwards, he writes. There may be some poetic licence here, perhaps.

Heath-Stubbs (1918-2006) wrote a poem called The Swift, and his portrait was painted by the Irish artist, the late Patrick Swift, a friend of John McGahern's who, incidentally, called him "the Bird Swift". Patrick Swift later established a pottery business in Porches in the Algarve in Portugal.

BirdWatch Ireland, the British Trust for Ornithology and the Northern Ireland Swift Group are collaborating in a geolocation project to plot the migration routes of Irish swifts and their wintering grounds in Africa.

Seven birds were tagged here last year at two nest-box colonies, one at Tubbercurry, Co Sligo and the other in Co Antrim. The geotagging is a first experiment here involving small birds. The technology involves a light sensor, memory chip, clock and battery, packed in a tiny waterproof tag - a tracking device in a backpack.

The tags don't transmit data so the birds' return is being eagerly awaited by devoted birders Michael Casey, Brian Caffrey and Mark Smyth of BirdWatch and NISG so that information on the chips can be downloaded. We must all be patient!

The 1914-18 war poet Edward Thomas described the swift as "shrieking in his fierce glee" with "wings and tail as sharp and narrow/As if the bow had flown off with the arrow."

They have often been described as "devil birds" as they scream through the skies like black missiles with those long pointed wings, tiny heads and beady eyes and four-pointed toes on each feathery foot to cling, intermittently, to a nesting crevice.

They cannot perch, mating in the air and snatching sleep while gliding in those thermals. As a schoolboy, I once found an adolescent bird in a field. I picked it up and saw its tiny feet and legs tucked underneath its body and placed it back on the ground. I realised it could not really stand up so I cast it aloft and away it sped. I hoped it tipped its wings back at me in thanks!

The birds appear more visible signalling their recent arrival, speeding over roofs, soaring to suck up to 10,000 tiny insects a day into their gaping mouths. But they have faced serious challenges seeking old nest sites on buildings traditional to them and finding them gone or wired off to keep out feral pigeons.

If you are enthusiastic, you could put up a nest box or two if you live in a tall old house or know a friendly factory owner - and (here's the tough one), play CDs of swift calls on a timer at dawn and dusk to attract them. All this will help to keep the breeding population steady at around 20,000 pairs.

The birds begin breeding soon after arrival in May and make a shallow nest of scraps picked up on the wing and bonded with saliva. Two or three young are hatched after 20 days and are fed continuously for a month until they take off. They begin the long trek to Africa in August.

Sunday Independent