The discovery of messages in raptors' nests has raised the possibility that many bird species encode signals into these structures, with seemingly decorative flourishes actually full of meaning.

Among black kites, scraps of white plastic are used to signal territorial dominance. To other kites, the scraps are a warning sign. To humans, they hint at an unappreciated world of animal communication.

"It's probably very common that other bird species decorate their nests in ways compatible with what we found," said Fabrizio Sergio, a biologist at the Doñana Biological Station in Spain. "And not only birds, but fish and mammals."

A few species, such black wheateaters and bowerbirds, are already known to use nest design in courtship displays. But such communication is considered uncommon and relatively one-dimensional, aimed entirely at finding a mate.

Other, ubiquitous forms of decoration – colorful ribbons and man-made trinkets, or flowers and bright grasses, found in the nests of hundreds of species – have been dismissed as insignificant adornment.

Likewise dismissed were the decorations of black kites, a medium-sized raptor common across Europe, Asia, Africa and Australia.

"Whenever you enter their nests, you'd see all these strange materials. Almost anybody that has started to study kites has talked about this," said Sergio. "But it was only five years ago that we started" looking at it carefully.

In a study published Jan. 21 in Science, Sergio's team describes its careful observations of 127 black kite nests in Spain's Doñana National Park.

They found that, several weeks before females laid eggs, birds festooned their nests with pieces of white plastic. Fitter birds, in possession of the best territory, tended to use more plastic. Weaker birds, with less-desirable territory, used less. Elderly and very young birds used none.

Territorial confrontations are common among kites, and proved closely linked to displays of plastic. Kites with much plastic in their nest were rarely challenged, while those with little were challenged daily, even hourly.

The plastic appears to be a convenient way of codifying and announcing strength, saving kites from indiscriminate or ill-chosen battles. The birds also had access to green and transparent plastics, ostensibly preferring white because of its long-distance visibility.

When the researchers intruded, however, adding extra plastic to nests, challenges paradoxically became more frequent – suggesting that kite community is aware of existing claims, and quick to test would-be cheaters.

Sergio is currently conducting a long-term study of nest decoration, hoping to see how it changes over the kites' multi-decade lifetimes. He would also like to know the origins of nest decoration, and whether – like birdsong – it has both genetic and cultural components.

Meanwhile, other ornithologists might look for messages among more bird species. Sergio stressed that territoriality is only one type of message; there could be many more, and each needs to be studied and interpreted.

"There are countless examples of storks and magpies and crows and swans decorating their nests," he said. "We think that they're not taking some funny material to their nest, but signaling other individuals."

Images: 1) Black kite nest./Fabrizio Sergio. 2) A black kite pair./Fabrizio Sergio. 3) A magpie nest festooned with ribbon and horsehair./minicooper93402/Flickr.

See Also:

Citation: "Raptor Nest Decorations Are a Reliable Threat Against Conspecifics." By F. Sergio, J. Blas, G. Blanco, A. Tanferna, L. López, J. A. Lemus, F. Hiraldo. Science, Vol. 331 No. 6015, January 21, 2011.