When the white swallows come back to Tacumshin

If I were going there this weekend – and I’m sorry I’m not, it’s too far – we would end up taking the road from Wexford to Kilmore Quay and then, after 12 km, a left for Tomhaggard. A right turn in the village and then another, after a kilometre, would brings us to Lingstown Corner and a car park. The sprawling lagoon of Tacumshin Lake would now be revealed, together with all the birders bearing binoculars and telescopes as they tramp to its furthest pools and bays.

Penned in by a shingle bank at Ireland’s southeast corner and furnished with reedbeds for roosting, Tacumshin in autumn is a prime staging post for thousands of Ireland’s migrating swallows, house martins and sand martins. They flicker above vivid assemblies of ducks and flocks of feeding waders, among them the regular rarities of vagrant US sandpipers.

Eric Dempsey was there the other day, watching an American buff-breasted sandpiper, one of his favourite birds, diverted from its distant route from the Arctic to Argentina. However then his eye was caught by something even rarer and lovelier: a pure white swallow, young and perfectly plumaged – his first in a whole career of birding. This has included two documentaries on Ireland’s swallows, for radio and television, in which he followed their migration to South Africa.

From these it became clear that young swallows leave Ireland first – but pause rather a lot on the way to reconcile their gene-plan with the landscape and imprint the map in their heads. The more experienced adults, with at least two migrations under their wings, actually overtake their young to arrive in South Africa first.

At Tacumshin, Dempsey's own instincts prompted several quick photographs, from one of which my drawing was made. His encounter meshed remarkably with other recent reports of white swallows to Eye on Nature.

The first was from Tom Ryan, a farmer near Cashel, Co Tipperary, who sent photos of an adult white swallow, with dark head, flying into his shed to feed nestlings, and later of an all-white juvenile swallow (no streamers to its tail) flying around his farm. Then came another photograph, from Eithne Mackell of Ballyduboy, on the Wexford coast, of a white juvenile perched on a television aerial – the same young bird, perhaps, that moved on south to Tacumshin.

All this fits together rather well and make it seem at least possible that the parent swallow, white with dark head, could appear at Tacumshin this weekend.

Migration is far from over: third broods are not uncommon and the adults often move out long after the young have gone – they may leave as late as November.

In all the photos, the swallows’ eyes were dark, which assigns their plumage whiteness to the genetic abnormality called leucism. This leaves out the cells producing melanin pigments from all or parts of a bird’s feathers, affecting colours as vivid as the robin’s breast, but it leaves black in soft parts like the eyes.

Albinism, on the other hand, extremely rare in the wild, robs birds of melanins completely, leaving eye tissue transparent, pink with blood, and with sight often fatally impaired.

White swallows have not been entirely exceptional in Ireland, judging by readers' reports to Eye on Nature. There have been five separate and well-scattered sightings over the past half-dozen years (or could members of the same family have been spotted singly at different points and times?) Abnormal whiteness is rated a handicap for birds, making them an easier target for predators or leaving them more prone to rejection in courtship or even fellowship on the telegraph wire. Repeatedly successful breeding and migrations in one family would be remarkable, as is the Cashel case.

Most naturally, all-white birds, as it happens, are big enough to take care of themselves – think of swans, gannets and the Arctic’s snowy owls. Others live on the ocean, where white underparts help the birds to hide from fish who see them, when at all, against a bright sky.

Leucism, nonetheless, affects a very wide range of species: the commonest is the blackbird. I’ve seen one wholly white-feathered specimen, in a thorn bush up the coast, but readers have reported a dozen, along with white or white-patched examples of mallard, sparrow, jackdaw, song-thrush, oystercatcher, magpie, greenfinch, shag, hen harrier and pheasant. Surveys in Britain have added a host of other garden birds, from goldfinch and dunnock to bullfinch and blue tit.

As for causes, garden bird tables and feeders have been blamed for distorting avian diets, depriving blackbirds, in particular, of the protein in neglected insect food.

Diet can certainly affect plumage – the pink of flamingoes, for example, is due to carotenoids in their natural aquatic food – but chance mutation is quite enough to explain the varied multitude of birds of a different feather.