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, writing in the journal Science, 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.