The white plastic bags fluttering in the treetops of the Italian Alps intrigued Fabrizio Sergio. The Italian ecologist knew the trash hung from the nests of a certain bird, the black kite. But why?
Many species of birds decorate their nests to attract a mate—but kites already have partners when they build nests. Still, the ornamentation on kite nests suggests “that there’s something they want to show off,” says Sergio, who works for the Spanish National Research Council.
As he and other scientists study the makeup of birds’ nests, they’re looking for signs of human influence. Some birds have begun using insulation, foil, and cigarette butts, for example, instead of natural materials, says Luis Sandoval, an ornithology professor at the University of Costa Rica. These adaptations may increase their reproductive success—or indicate that natural building materials have disappeared from the habitat. “Humans are directly affecting birds’ nests in a way that we are still trying to understand,” Sandoval says.