Sex, Lies And Notebooks - behavioral patterns of the European oystercatcher

One of the world's steamiest soap operas plays for free on the Netherlands' coast and stars the European oystercatcher

Along the edge of a salt marsh on the Dutch island of Schiermonnikoog, a boldly patterned black-and-white bird suddenly attacks. The bird, a European oystercatcher that researchers have dubbed Sambo, has waited 11 years for this moment.

Sambo's adversary, Ludwig, is also a male oystercatcher. The two shorebirds chase each other across a tidal flat, grapple in the mud and grab each other's bill or wing. After about a minute of fighting, Ludwig has had enough and runs away.

Off to the side, a female oystercatcher named Greta has been watching. During the past winter her longtime mate died. Now the victorious Sambo, claiming his prize, can mate with her and take a prime nesting territory on the boundary between the mud flat and the salt marsh.

Schiermonnikoog, the setting for this combat, is on the edge of the Waddenzee -- a shallow body of water that during low tide becomes a vast mud flat stretching for about 5 miles to the Dutch mainland. This fertile tidal zone

harbors great numbers of mussels, cockles and other shellfish that oystercatchers and people, among others, harvest for food.

Since 1983, Dutch ornithologists have studied the avian dramas that take place here every spring and summer. By banding thousands of oystercatchers and noting their interactions, they've learned about the unexpected ways these birds try to assure their breeding success. They've found that for the oystercatcher, the line between love and war is very fine indeed.

Their study subject is one of the most common shorebirds along the North Sea Coast. Like its cousin, the American oystercatcher of the U.S. East Coast, it is black and white, with pink legs and a long orange bill shaped like an oyster knife. Noisy and numerous, the crow-sized birds are a hallmark of the Dutch shore where, in great profusion, they hammer open shellfish with their strong bills.

Nowhere do more oystercatchers live than on the Waddenzee. Of the world population, about one-quarter -- an estimated 200,000 birds -- winter here (others fly as far south as northern Africa). Of these, three-quarters leave in early spring for breeding areas in Scandinavia, Russia and Iceland, where winters are too harsh to allow shoreline feeding. The rest remain in the Netherlands throughout the year, and are the subjects of scientific scrutiny.

Biologists on Schiermonnikoog begin their annual labors in early spring, when most of the wintering oystercatchers leave and those that remain begin to defend breeding territories. Along the Waddenzee food is abundant and nesting territories are small; an observer in a blind can oversee dozens.

There are frequent noisy disputes along the edges of these territories. Often two pairs will run along their mutual boundary, heads down, uttering vigorous high-pitched piping sounds. Or the birds engage in spectacular "hovering ceremonies," in which as many as 15 oystercatchers slowly flutter over a patch of mud while piping. "It's one big neighbors' quarrel over the territorial boundaries," says Bruno Ens, who studies oystercatchers for the Dutch Institute of Forestry and Nature Research.

The biologists have been able to dissect those quarrels by placing colored leg bands on the birds. Detailed observation of these birds has revealed which ones breed, with which partners and where. A key early discovery was that chicks hatched in nests along the salt-marsh edge survive much more readily than those from nests inland. Birds on the edge of the marsh produce three times as many young, on average.

The reason soon became clear. Birds nesting inland may have to fly as far as two-thirds of a mile to forage on the mud flats, then back again to deliver food. "Birds nesting on the edge of the salt marsh just walk onto the mud flats and can give each prey item to their young immediately," says Ens. "The `leapfrogs,' as we called those that nest inland, have to work terribly hard but have little success. The `residents,' as we called those on the edge, seem to do so little but are more successful."

Leapfrogs, in other words, have to commute. They also have to defend two territories: one around their nest and one encompassing a feeding area on the mud flats. Both parents have to do this for a month before their chicks can fly. As a result, most of the leapfrog chicks starve. Others are taken by predatory gulls. Resident chicks, meanwhile, can readily hide from gulls along the eroding dirt of the salt-marsh edge, and they usually receive sufficient food from their parents.

It's ironic, Ens notes, that so many oystercatcher chicks starve within sight of the mud flats. After all, the vast numbers of oystercatchers that feed on the Waddenzee in winter have dramatically thinned by summer. Yet many chicks don't get enough food, even though the mud flats produce far more shellfish during the warm months. Some chicks probably starve because their parents are too busy defending territories to feed them. Ens says oystercatchers are "victims of a rigid territorial system."