MELISSA BLOCK, host:
The animal world has a vast vocabulary.
(Soundbite of chickens snorting)
BLOCK: These are male prairie chickens, common in the Midwest. They're strutting their stuff and snorting through enlarged air sacs.
(Soundbite of hammer-headed bats singing)
BLOCK: And these are male hammer-headed bats in Africa. They hang from trees along riverbanks and sing for attention. A lot of the signaling is about sex, as in, hey, I'm here and I'm available.
Well, scientists in Spain have discovered a very different kind of signaling. It's by birds called black kites.
As NPR's Christopher Joyce reports, they decorate their nests with plastic apparently to scare other birds off.
CHRISTOPHER JOYCE: It's not just any kind of plastic, it's only white plastic, bits and pieces the birds scavenge and lay inside their nests alongside their eggs.
(Soundbite of black kites chirping)
Biologists wondered for years why these big predatory birds do this. So a research team set up in a national park in Spain. They recorded and videotaped kites in 127 nests for five years. And slowly this decorating habit started to make sense. It was a signal by the strongest birds that they were tough and could protect their nests.
Now, a lot of animals do things to show either toughness or sexual allure. They'll dance or bellow or display some overgrown body part. But Fabrizio Sergio of the Spanish Council for Research says this is a pose of a different color.
Mr. FABRIZIO SERGIO (Spanish Council for Research): The most funny thing is probably that it's done in this case and not through some parts of the body such as the plumage or a very pretty song, but it's made through some objects, which are collected and constructed within the nest.
JOYCE: Sergio and his team think it's a construction that signals bravado - think Harleys and tattoos. But the bravado is real because only the stronger, tougher nesting pairs decorate this way. The weaker birds, the youngest and the oldest, do not. In fact, they actually rejected the plastic when scientists added it to their nests.
Mr. SERGIO: Some of the individuals actually did not want it at all, they immediately removed it, and this was very extraordinary.
JOYCE: One reason could be that when the kites add plastic, their nest immediately draws attention and attacks by other birds looking for food or a new nest. The tougher birds fight them off, and after that, they're pretty much left alone. Steve Nowicki is an evolutionary biologist at Duke University.
Mr. STEVE NOWICKI (Evolutionary Biologist, Duke University): You know, a male who is in his prime is going to put more decorations on because he can back up his signal, it's not a bluff.
JOYCE: But weaker birds don't have the wherewithal to fight a lot.
Mr. NOWICKI: The young males or the older, more senescent males do it less because they're just less able to be successful if there is an aggressive contest that follows.
JOYCE: And apparently they know it. That's why the weaker birds threw out that plastic when the Spanish scientists added it to their nests. The birds realized it was false advertising that could lead to trouble. Nowicki says that's no mean feat.
Mr. NOWICKI: That requires some capacity to self-assess, realize that the signal that you find on your nest is off and then adjust it.
JOYCE: Nowicki says this signaling behavior works because the attacks keep weaker birds from bluffing. It means in scientific terms that the white plastic signal, I'm tough, is honest.
But wait a minute. Is it really wise to advertise where your eggs are? One bird biologist, Gerry Borgia at the University of Maryland, says he doesn't buy the toughness-signal idea.
Mr. GERRY BORGIA (University of Maryland): To make a nest more obvious seems extremely strange.
JOYCE: He thinks the kites may be doing something quite different with the plastic.
Mr. BORGIA: Camouflaging eggs.
JOYCE: Eggs are white. The kites choose only white plastic. Maybe an array of white plastic in a nest is simply there to confuse predators that eat eggs.
The kite study was published in the journal Science, and it may take more studies to confirm what's going on here. But for biologists, this is more than just bird weirdness. It's a clue to how evolution winnows out winners from losers.
Christopher Joyce, NPR News.