Thousands of birds an hour

The solitude of the strand is shared these mornings with the sandwich terns, fresh in from West Africa. The sun tries to pin their forked shapes to the ocean, dazzling white against the blue, but they dance and swoop like paper kites, hovering and plunging after sandeels where the waves begin to curl.

Heard across the surf, their call is almost musical, but they are really the noisiest as well as the biggest terns we have. On their tiny island along at Cross Lake, dammed in behind a pebbly shore, the massed chorus of "kirrik! kirrik!" drowns out even the scolding of the black-headed gulls.

It's when the terns arrive in spring that I wish we lived even further out on the edge - near the tip, say, of one of the headlands that thrust into the ocean. So much goes on out there, among the birds and whales. This is the season when a lighthouse, or one of those high cliffs with nothing around it but sea and sky, would offer a balcony from which to view quite staggering processions of birds.

At Cape Clear, for example, off Baltimore in west Cork, a dawn sea-watch over the past few weeks would have found razorbills streaming past the island in V-formations, sometimes of 10,000 birds an hour (and often, as watchers have noted, with a solitary puffin as leader). In early July, when the Manx shearwaters are bringing food to their young in their burrows on the Blaskets, they fly east past Cape Clear in the morning and back to the west in a torrent at dusk, up to 30,000 an hour. Terns, storm petrels, kittiwakes, guillemots, gannets - a dozen different species come flooding past our coasts, strung out in long lines, whirring in bunches, skimming on stiff wings, fluttering low over the waves. It's a spectacle worth getting wet for, and the best weather for sea-watching is some of the worst: a murk of wind and drizzle makes the birds hug the coast. Sometimes the gannets pass so close that they turn their heads, to note the figure crouching on the cliff.

Some pattern to these huge movements of birds began to emerge exactly a century ago, in the notes that lightkeepers made for the great Irish ornithologist, Richard Barrington of Bray. His analysis of almost 20 years of records, published in 1900, remained the standard work on Irish bird migration until the first observatories, Cape Clear among them, opened in the 1950s.

Since then, more and more scientists have put to sea to explore the shifting world of birds beyond the reach of telescopes, and find new explanations in the realm of oceanography.

They have highlighted, for example, the great summer concentration of seabirds, whales and dolphins off the south-west of Ireland and around the Hebrides, off north-west Scotland. Indeed, though Scotland and its islands can boast vastly bigger breeding colonies of seabirds, the highest densities of birds at sea are off the coast of counties Kerry and Cork.

It's particularly here, at the edge of the continental shelf, that the welling up of deep, cold water brings minerals to the surface, nourishing the growth of plant and animal plankton. This is reinforced near the coast, where warm Atlantic currents surge in to the long headlands and mix with the colder, coastal water.

The plankton "fronts" of summer (shown distinctly in infra-red images from satellites) provide abundant food for fish and thus for their predators. Gannets plunge beside porpoises; shearwaters dive among minke whales; dolphins herd fish to the surface, where they virtually leap into the beaks of birds.

This rich supply of food sustains the big bird colonies ashore, from 22,500 gannets clamouring on the peak of Little Skellig and 20,000 pairs of Manx shearwaters on Kerry's Puffin Island, to the guesstimated hosts of storm petrels flickering in at midnight to feed their young on uninhabited islands from Great Skellig to Donegal.

Almost nobody actually sees these little petrels at their nests: one can even forget they exist. Years ago, camping alone on Inishvickillane, I pitched my tent in the lee of a mossy dry-stone wall, and the petrels, returning like bats in the dark, cannoned into the fabric and scrabbled down it, startling me out of my wits.

It is the storm petrel's use of rocky crevices (rather than separate burrows, like the puffins and Manx shearwaters) that makes them peculiarly difficult to count. The only sign of their presence, in daytime, is the mysterious churring, crooning and hiccupping that drifts out from the rocks; grand estimates for the species have put the combined British and Irish total at anything between 70,000 and 250,000 pairs.

This imprecision has tantalised our ornithologists, especially as Ireland seems to be the one true home of this wandering, Atlantic species. It also makes it difficult to judge the success of conservation, as our islands come under increasing pressure from recreational use.

The first "complete" count of storm petrels will be part of the Seabird 2000 census to be carried out round the coasts of both islands, and a survey team from BirdWatch Ireland has carried out some often difficult and dangerous fieldwork on precipitous cliffs and scree slopes, testing special census methods in advance.

When the petrels are incubating their eggs, most of them respond very readily to a tape playback of the male's purring call - most, but not all, so that the fine-tuning of mathematical probability models needed four or five repeat surveys during the peak egg-laying and incubating periods. And these periods themselves could vary, depending on location and climate, even between Co Kerry and Co Mayo.

The fieldwork, by Ian Beatty, Dave McAdams and Dominic Berridge, has shown how adaptable the petrels can be in searching out suitable crevices on these bare and windy islands. On Puffin Island, a BirdWatch reserve, they nest largely on scree slopes high above the sea. On Inis Tuaisceart, in the Blaskets, they hide under clumps of heather. On Inishglora, off Mayo, their habitats range from dry stone walls to holes under boulders and among the rocks of storm beaches.