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Nest Garbage Says, 'Keep Out!'

What's garbage to most people is the perfect nest decoration for some birds. Black kites adorn their nests with bits of white plastic they find littering the ground. Although most birds decorate their nests to woo mates, the kite's adornment says, "Don't mess with me." Researchers have discovered that these humanmade objects warn off rivals, signaling that the nest owner is willing to put up a fight if challenged for its territory.

Ornithologists have long marveled at the ability of certain birds to add sticks, shells, and berries to nests and other structures to impress mates. Among black wheatears, a small songbird found in North Africa and Spain, for example, males show off their muscles by the size and number of stones they incorporate into their nests. And well-decorated stick structures called bowers signal that a male bowerbird is able to keep other males from stealing its ornaments.

But some birds add objects to their nests for no apparent reason. Male black kites—a medium-sized bird of prey that lives in Europe, Asia, Australia, and Africa—embellish their nests with litter, but they don't do it to attract females, as both genders build nests together. Fabrizio Sergio, an ecologist at Doñana Biological Station in Spain, wondered whether the birds might be using these decorations as signals of another sort. He and his colleagues tested their idea on black kites that have been banded and followed for years in a marsh in southwestern Spain.

The researchers cataloged and quantified the nest decorations of 127 nests, monitoring the survival of the nest owners and their young for that season to determine whether nest decorators were the fittest of the flock. They also walked along nearby roadsides to find out what garbage was available and compared that with what the birds actually placed in their nests. In one experiment, Sergio's team set out green, transparent, or white, 20-centimeter-square sheets of plastic to determine which ones the birds preferred. In another test, the researchers added more decorations to certain nests and evaluated how those additions affected the behavior of other birds.

The birds were particular about their decor, bypassing green and transparent pieces of plastic for the bright white stuff that could be seen from far away. The fitter birds, which survived better and produced the most young, had nests closer to their marshland hunting grounds and tended to put up the most decorations, whereas birds with poorer territories had none. Very old and very young birds also had plain nests.

Thus, nest decorations were correlated with how fit the birds were. And potential intruders took notice. Birds interested in stealing food or commandeering a nest almost never bothered the most adorned nests, but plain nests could undergo up to six raids per hour, Sergio and his colleagues report online today in Science.

Timothy Wright, a behavioral ecologist at New Mexico State University in Las Cruces, thinks this may be the first study to show that organisms use nest decorations for more than just courtship. "It's a very thorough demonstration that such signals may function in the context of territory defense," he says. "I think few ornithologists will look at nests in quite the same way again."

Nor should they, adds Sergio:"We do believe that hundreds of species could be using their nests not just to contain or protect their offspring but also to convey information about the builders."