As the typhoon season cuts between summer and autumn, many species are on the move. This is the season of migration for land birds and seabirds. While the land birds island-hop between Northeast and Southeast Asia, some of the seabirds are embarking on journeys that may span entire oceans. Streaked shearwaters around Japan, for example, will be heading off as far as Australia.

But just what is a seabird?

Although there are around 9,700 species of birds on earth, only about 300 are considered to be seabirds. In one sense, then, they are few.

Yet the majority of bird species, around 9,400, inhabit the land, which amounts to just 30 percent of the globe. In effect, 3 percent of the world's birds, the seabirds, inhabit 70 percent of our planet.

The seabirds are also special because although they have the watery 70 percent of our planet to roam over, they are tied to the land (or fast ice) for one crucial part of their annual cycle: the breeding season.

Seabirds are remarkably diverse. Some are flightless, such as the Galapagos cormorant, whereas others, such as the albatross, are masters of flight. They vary in size from emperor penguins weighing up to 40 kg, down to the tiny diving petrels and storm petrels, which may weigh as little as 25 grams.

Seabirds also vary enormously in their physical structure, and in the ways they are adapted to their exploration of the oceans and to the use of land for breeding. Adaptation in any direction brings both benefits and limits, so, for example, while the shearwaters and albatrosses are the masterful gliders of the seabird world, they cannot dive. They may travel over thousands of kilometers of ocean, some circling the entire Pacific on their annual migrations, yet their light structure and their enormously long wings in proportion to their bodies mean that they cannot submerge, so they must feed from the surface.

At the other extreme are the dumpy diving seabirds such as the auks. They are so well adapted to underwater hunting that their flying ability is seriously compromised. Their wings propel them very effectively under water to chase fish, but must whirr at high speed to keep them airborne. They are at an interesting stage of adaptation, a kind of Catch 22. If their wings were to become more effective for underwater hunting, they would be less effective in the air. Conversely, if they became better fliers they could reach their breeding colonies more easily, but would be less able to catch food under water.

The penguins have taken the auk model one step further and opted for complete flightlessness, and their rudimentary wings are now highly efficient underwater oars. The cormorants, which also dive, have taken a very different approach. Instead of using their wings in both media, they use their wings in the air, and underwater they use their enormous, paddlelike feet.

For all their differences, there is one crucial thing that all seabirds have in common: the physiological ability to process and excrete the excessive amounts of salt in the food they eat and the water they drink.

After all, they live in an environment in which we could not possibly survive. The term "tube-nose" used for certain seabirds refers to a special gland above the base of the bill that removes salts from the body fluids. Birds such as the albatross, shearwater and petrel excrete this salt as a concentrated fluid, which drips down the tubes on top of the bill.

Scientists divide the seabirds into four large orders: the Sphenisciformes (17 species of penguins); the Procellariiformes (113 or so tubenoses); the Pelecaniformes (50 species of pelicans, gannets, tropicbirds, cormorants and frigatebirds); and finally, the Lariformes (about 110 species of skuas, gulls, terns, noddies, skimmers and auks).

Let's think of them, however, in terms of their lifestyles. Gliders, including albatrosses, shearwaters and large petrels, are able to use the wind to shear and glide long distances without flapping very much. They take their food from the surface of the water. Aerialists are pelicans, gannets, tropicbirds, cormorants, frigatebirds, gulls, terns and small petrels. They are capable flyers, but must flap their wings often to remain airborne (though on migration enormous birds such as the pelicans can also soar). These birds take food from the surface and down 1 or 2 meters from the surface.

Finally there are the divers, which includes all penguins and auks, which fly poorly or not at all and dive deeply for their food using their wings to "fly" under water.

Let's look first, then, at the gliders. Most of the albatross species occur in the Southern Hemisphere. The largest among them are the wandering and royal albatrosses, with wingspans of 2.5 to 3.5 meters. They are at home in the Roaring Forties and the Screaming Fifties, and circle Antarctica, breeding on the various sub-Antarctic islands.

The only tropical species is the waved albatross, which is confined to the Galapagos Islands and the surrounding seas. There are no albatrosses in the North Atlantic, and here in the North Pacific there are just three species: the Laysan, black-footed and short-tailed albatross. The short-tailed breeds almost exclusively in Japan; its breeding grounds on Torishima, with about 600 birds, are particularly famous.

The commonest of the North Pacific albatrosses, numbering close to 360,000 pairs, is the Laysan, which breeds on islands in Japanese waters, in the Hawaiian Islands and even on islands off the coast of Mexico. It ranges widely in the North Pacific and is particularly common in Japanese waters, where it is frequently seen offshore from ferry boats.

Among the many other tubenoses regularly seen in Japanese waters, the commonest, after the locally breeding streaked shearwaters, are the sooty and short-tailed shearwaters, which migrate past Japan annually.

Back in February, I made my first visit to the Snares Islands of southern New Zealand and was amazed to find that as many as 2.74 million pairs of seabirds breed there. Most of them are sooty shearwaters. The short-tailed shearwater breeds mainly around southern parts of Australia and especially Tasmania. Some 23 million of them migrate to the North Pacific for the months of April to September, where they spend the northern summer (their winter) feeding. Then they are very common, but by now most are heading south again to breed.

Though superficially similar, albatrosses and shearwaters have very different nesting ecologies. Whereas albatrosses nest in colonies on exposed open flat areas where winds are common, the shearwaters are burrow nesters, excavating their nesting holes in soft soil on top of islands. As hole nesters, shearwaters are particularly at risk from introduced rats, which eat their eggs and young.