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Two Birds, One Plume

Come spring, the woods are full of apparent nonsense. All over the world, male birds sing, flash bright feathers, flaunt fancy plumes, build structures, and even dance to lure potential mates. Noise, color, and motion are all great ways to attract predators; singing and dragging excess feathers through the air take extra energy. A sensible female bird shouldn't look twice at a male guilty of such high-risk behavior. Who'd want to chance raising a family with an energy-wasting, danger-luring showoff?

Just about every female bird on the planet, that's who. They flock to the showoffs. If they have a choice, usually they'll pair up with the showiest male they can find.

Evolutionary biologists have debated for years about why apparently negative features should be judged so positively by female birds. According to the British journal Nature, three hypotheses dominate the argument.

One holds that the features are essentially arbitrary signals; according to this view, a loud and complicated song, for example, just happened to be what a given species came to use to distinguish itself from similar species. The loudest singer is simply sending the strongest signal.

Another theory holds that certain characteristics won out over evolutionary time because they actually are direct indicators of their owners' quality. By this truth-in-advertising view, a loud, complex song shows that the singer is in good physical (and mental, for a bird brain) condition. The third theory is known as the handicap principle; it assumes burdensome traits show that their possessor is a mighty fellow capable of overcoming the difficulties they cause him. A loud song thus brags about the singer's ability to avoid predators attracted by the song.

The author of the Nature article, Jared Diamond, thinks evidence recently gathered from the cloud forests of New Guinea has clarified the debate. The information came from a pair of ornithologists named Frith, who studied a little-known species of bowerbird.

In avian terms, male bowerbirds build palaces--big, complex structures of interwoven twigs and leaves, decorated with pebbles, snail shells, or other objects that suit the bower-builder's tastes. These aren't nests; they're showplaces, true bachelor pads for luring females.

The Friths observed male Archibald's bowerbirds, noting especially how the birds decorated their bowers. The most cherished decoration proved to be particular feathers from another kind of bird--the head plumes from the King of Saxony bird of paradise. These feathers may be the oddest sported by any bird; as "Nature" put it, the two plumes resemble "a fine wire bearing dozens of squares of blue plastic" and are several times the thrush-sized bird's length. Bowerbirds possessing these purloined plumes gave them featured places. If the Friths moved them to less conspicuous spots, the birds would quickly return them to prominent display.

Diamond considered this observation. What message is conveyed to bowerbird females by the presence of the bird of paradise plumes? The plumes are hard to get: the King of Saxony bird of paradise is an uncommon species, lives in dense forest where dropped feathers are hard to find, and sheds its plumes only once a year. Therefore, Diamond believes, the plumes signal the skills and abilities of a superior bowerbird: he found these rare objects and brought them home. The Friths' study supports the truth-in-advertising hypothesis. That may work also for another species that makes decorative use of the plumes. Men of the New Guinea highland tribes incorporate these bird of paradise feathers into their headdresses.

What about the bird that grows the plumes? The handicap principle may operate here. Only a superior bird could navigate through the dense cloud forest trailing pennants from his head, and the larger the plumes, the more difficult the trip. So, for both originator and borrower, the plumes are conspicuous symbols in an ancient sign language--and they mean Better Bird Here, as the females can plainly see.