Plastic-Lined Nests Keep Rivals at Bay

Not just any bird can pull off the nest-full-of-shredded-shopping-bags look.

Among black kites nesting in Spain's Donana National Park, breeding pairs in the prime of life collect an abundance of white plastic bits and tuck them into their nests, says ecologist Fabrizio Sergio of the national research network CSIC's Donana Biological Station.

With observation plus some home makeovers, Sergio and his colleagues conclude that this conspicuous plastic decor warns rival birds that any attempts to take over a desirable territory will meet fierce resistance. The jumbles of pale, easy-to-see oddments apparently serve as a reliable indicator, the researchers say in the Jan. 21 Science, because weaker birds seem loath to display phony warnings.

Fights between black kites (Milvus migrans), mid-sized birds of prey, can get quite violent. Combatants sometimes even lock talons in midair, twisting and struggling as they plummet toward the ground. Signals of strength might be valuable in picking which fights to start, Sergio says. "If you're a brown belt, maybe you don't want to challenge a black belt, and you're happy to know it beforehand," Sergio says.

The power of bird nests to send signals has been "greatly underrated," comments Michael Taborsky of the University of Bern in Switzerland, who wasn't part of the research team. Just as feather colors or other features of an animal's body can send messages, so can structures that organisms build.

Donana was a great place to study the function of plastic-bedecked nests, Sergio says, because a long-running research program had established the ages of the birds. Tracking 127 kite nests, researchers found that extensive nest decoration showed up mainly among birds between 7 and 12 years old, in their physical prime. Younger birds, or ones between 13 and 25 years old  displayed little bling or none at all.

Researchers linked the tendency to decorate with domestic success among kites. Pairs with bigger displays were increasingly likely to rear two or three chicks a season instead of just one. Also, nests with a lot of decoration tended to have fewer intruders.

To see what might be important to birds as they collected their doodads, researchers set out shreds of white, green or transparent shopping bags. Out of the 33 pairs that picked up some of the offerings, 29 chose only white snippets. Green or even transparent plastic would have been less conspicuous to chick-snatching predators, Sergio says, so the team concluded that birds preferred something bright and visible.

When researchers added plastic to a range of nests, the sudden uptick in finery actually increased the number of attacks, at least for the first few hours, on nests that had previously been unadorned. Sergio interprets the onslaught as "an initial social test," in which intruders check to see whether defenders really can live up to their plastic. This testing could help keep the signaling system honest, researchers say.

The possibilities for animals using shelters as signalling devices stretch far beyond birds, even to mammals, comments Mike Hansell, emeritus professor of animal architecture at the University of Glasgow in Scotland. "The richest selection of mammalian builders is among the burrowing rodents," he notes. "Are any female rodents overawed by labyrinths built by males?"