Bonxies are new bird bullies on the block

Another Life: At low water, in a calm sea, reefs surface off the shore like wallowing sea-dragons. As the tide rises they vanish again, or are betrayed by little riffs of snowy surf along their spines.

On any trip to the islands, they remind the occupants of a rubber dinghy (or "rib" as I must learn to say) of the essential fragility of its skin. Indeed, as the ocean darkens beneath the skimming bow, one looks back at the Tolkienesque frieze of Connacht's mountains and feels, needless to say, very small, but also in a state of unnatural suspension: a travelling cocoon of space between sky and sea, definable only by GPS co-ordinates.

The islands along my horizon contain three big ones - Inishbofin, Inishturk and Clare - and many others, uninhabited and often difficult to land on, whose ownership is annexed to island and mainland holdings. They may or may not now be grazed by sheep, but at their lofty western reaches these windswept wedges of land are the preserve of nesting seabirds.

At the south-west of Ireland, in richer waters, such island cliffs offer seabird spectaculars: guillemots and razorbills stacked on the ledges, puffins wheeling out of their burrows, gannets turning whole islands white.

Things calm down a bit as one travels north, and ornithologists seek more esoteric satisfactions - seeking out the rare Leach's petrel, say, on chilly nights among the sea-pinks.

It was in daylight, however, in the summer of 2000, that fieldwork volunteers in a mammoth seabird survey of the Irish and British coasts had a thrilling, if briefly alarming, experience. On one of the islands along my horizon, they found themselves dive-attacked by a pair of fiercely burly brown birds, snarling a strangled kayaya! Great skuas, famous for clouting the heads of intruding ornithologists, just shouldn't have been there.

They are normally seen by Irish birders only through telescopes, in ones and twos in offshore processions of seabirds migrating south in autumn. Now here they were ashore in summer in a full territorial tizzy. In 2001 a nest was found, complete with downy chick. "Great Skua Catharacta skua breeding in Ireland," announced a paper in Irish Birds, prepared by BirdWatch Ireland's Stephen Newton, the RSPB's Alastair Moralee and ornithologist David Cabot (a neighbour of mine on the Mayo shore). Moreover, they reported, skuas had been seen on a second island, 20km from the first. It was to this that the rib was headed, with David Cabot at the tiller.

What gives the great skua its dire and piratical reputation? It's a kleptoparasite, gaining much of its food by robbing other seabirds, chasing them nose to tail and bullying them into dropping - or messily disgorging - the fish they have caught. While physically much of a size with the herring gull, the skua's bulky, barrel shape and fast, agile flight give it the aerial menace of a much bigger bird. It not only chases others but sometimes kills them: a skua drowning a black-headed gull was watched in the Shannon Estuary, while the murder of a herring-gull was seen at Inishbofin.

Such violence seemed quite at odds with the atmosphere of the island we came to (anonymity is still de rigeur at this stage). Basking harbour seals barely stirred at our approach to a boulder cove jewelled with bright blue jellyfish. Oystercatchers and black-backed gulls screeched and cackled in a way that only emphasised the very special, echoing absence of human sounds.

At the top of the island, where a great skua stood on big flat feet, there was only the sough of a gentle summer wind.

Binoculars insisted on a bird at least twice its actual size, so coldly regal was its bearing. If Charles Haughey could silence a room just by walking into it, the great skua had something of the presence of the sea eagles that he tried to install on his island. When it took to the air, white flashes in broad wings suggested another raptor, the buzzard, with which it can be confused. Keeping our distance, we did not invite attack, though the skua's settled watchfulness suggested a (lifelong) mate sitting on a nest somewhere close. Aloft, it was mobbed by a pair of black-backed gulls, a routine harassment shrugged off and then taken out, in turn, on a passing island raven. We retreated, well satisfied to confirm a continuing presence.

The "bonxies", to give the great skuas their Hebridean name, are now a fact of Irish bird life, bringing the species 700km further south from its breeding colonies on St Kilda, Shetland and Scotland's western isles. Such expansion by a basically sub-Arctic bird comes at a time when southern European birds, such as the little egret and Mediterranean gull, have established themselves in Ireland. As the authors of the Irish Birds report suggested, it "runs against most scenarios of the effects of global warming". It could also mean that our island birds will have to cope with a new bully on the block.