White plastic discarded by litterbug humans is helping to preserve the pecking order among a bird population.
Scientists studying black kites in Spain discovered that strong birds in the prime of their lives decorated their nests with large amounts of the rubbish.
Very young and elderly birds, who were less able to defend their territories, hardly decorated their nests at all.
Researchers believe the behaviour provides a form of signalling that ensures no misunderstandings.
Weaker birds do not want to appear tougher than they are – while the best fighters wish to command respect.
When the scientists added extra white plastic to nests, the birds wasted no time in removing it. Failure to do so quickly invited trouble.
The findings, from a study conducted at Donana National Park in southern Spain, are published in the journal Science.
Territorial battles between nesting kites and “floating” invaders were common, said the researchers. Breeding pairs tended to collect their nest decorations one to two weeks before laying. They seemed to prefer pieces of white plastic over other materials and colours.
Dr Fabrizio Sergio, from the Spanish state-run research institution, CSIC, and colleagues wrote: “The objects seem placed so as to be highly visible and are collected during the peak period of territory trespassing and long after pair formation.
“Our results show that nest decoration is a graduated signal that conveys complex information on territory quality, individual viability, and dominance in social interactions.”