Plastic adorns the nests of birds fit for a fight
It might not seem de rigueur but for a black kite furnishing one's nest with white plastic is a major statement.
Spanish scientists have documented how this bird of prey will decorate its nest with large amounts of rubbish.
It is a symbol of success, apparently - the biggest collections of plastic are displayed by the black kites with the most chicks and the best territory.
The research, conducted in Donana National Park, is reported in this week's edition of Science magazine.
The Spanish team behind the study says the strips, mostly from old bags, are a signal to other birds that the incumbent will put up a fierce fight if any rival tries to move in on the local patch.
"People who've worked with black kites and even red kites, their cousins, had noted these birds' nests were often littered with rubbish, but this is the first time the function of this decoration has been studied," said Dr Fabrizio Sergio.
"It is not only white plastic - they surely prefer that - but they can actually use a wide range of materials, including cloth and paper," he told BBC News.
The 500-sq-km Donana National Park, a Unesco World Heritage Site, is famed for its wildlife, which numbers the Iberian lynx and imperial eagle among its inhabitants.
It is also home to hundreds of pairs of black kites (Milvus migrans), which are medium-sized migratory raptors.
These birds are known to fill their nests with man-made rubbish, and a breeding pair will start scavenging for the litter about 20 days before laying eggs.
Dr Sergio and colleagues from the Donana Biological Station wanted to establish if this activity represented a form of communication and, if so, what it was saying.
Over a period of five years, they observed the behaviour of the kites and the nature of their nest decoration.
They even intervened on occasions to change the adornment to see how the birds would react.
The team found that the strongest birds in a middle age bracket were the ones with the most plastic. The very young and the very old had hardly any at all.
The birds with the most white plastic were also the best at defending their territory and monopolised the food bait laid by the researchers.
"These aggressive birds are probably the ones in best body condition and this probably coincides with the age class which we defined as the prime age, which is between seven and 12 years old," explained Dr Sergio.
"This is the age class that shows the best reproductive success and the best survival. They are the most viable individuals.
"The white plastic clearly functions as a threat to other individuals of the same species - to other kites. It's basically a 'keep away, no trespassing' signal. A comparison with humans would be the notices placed on the gates of certain nice houses which say 'beware, guard dog'.
To test its ideas, the team made additions to the nests of kites that had hardly any plastic.
Almost immediately the owners of those nests started stripping out the plastic.
The birds knew that to make an extravagant display would invite challenges, said Dr Sergio, and very young or elderly birds would not risk picking fights they could not win. "Cheating is punished," he added.
Other species in nature will add objects into their nests. Examples include not just man-made items, but stones, green plants, snake skins, and even dung.
The signalling function of such activity may not have been fully appreciated, the Spanish team believes.
The Donana Biological Station is funded by Spanish National Research Council (CSIC).