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Birds let their nests speak for them

Decorated nest of an eleven-year-old black kite. At this age, kites typically decorate their nest exuberantly.
[Photo F. Sergio]

Researchers studying black kites—medium-sized birds of prey—have discovered that the ways in which the birds decorate their nests can speak volumes to other birds in the area. Apparently, the black kites that decorate their nests with the largest amounts of white plastic are also the best fighters. Plus, they produce the most offspring and live in the best territories.

Normally, about 20 days before laying their eggs, male and female black kites begin scavenging their environment for items to decorate their nests with. The birds prefer decorating their nests with pieces of white plastic and, in fact, they seem to avoid other materials and colors.

A seven-day-old black kite nestling seeks the cover of its mother in a decorated nest.
[Photo F. Sergio]

To make their discovery, a group of researchers led by Fabrizio Sergio and Julio Das monitored the decorations of 127 black kite nests in Dońana National Park in Spain. These researchers performed a series of experiments with the black kite nests and observed that the strongest birds, who were seven to 12 years old, decorated their nests with lots of white plastic. Very young and elderly birds, on the other hand, hardly decorated their nests at all.

The researchers found that black kites with the most white plastic in their nests were also the most capable of defending their territory from other intruding black kites. And it doesn't seem like the birds are interested in pretending to be something they are not: When the researchers added more plastic to the birds' nests, most of the black kites removed it immediately.

These researchers suggest that the birds do not want to pretend to be tough if they really aren't—much like a new student in a karate class would not want to pretend to be a black belt on the first day. The researchers' findings indicate that structures built by animals might serve as signaling devices—like the black kites' nests—more often than researchers have believed in the past.


This research appears in the 21 January 2011 issue of Science.