You might raise your eyebrows at their choice of material, but you wouldn't want to pick a fight with a black kite whose nest is adorned in white plastic, Spanish researchers said Thursday.
By examining 127 nests of the birds of prey in Spain's Donana National Park, researchers reported in the journal Science that the strongest birds showed their ferocity by decorating their homes with lots of bits of white plastic.
And only white would do, said scientist Julio Blas of the Spanish National Research Council.
"The amount of decoration is related to the fighting ability of the individuals," Blas said.
Birds of prime age, seven to 12 years old, tended to use the most ornamentation.
"They prefer plastic, and they specifically prefer white plastic which makes the nest more visible not only to humans but to other kites."
The extra decor, which tended to accumulate about 20 days before the female laid her eggs, seemed to invite more clashes with other aggressive, highly territorial birds.
But those who showed the most white plastic in their towering branch nests were "also found to be the most capable of defending their territory from intruding black kites," said the study.
As an experiment, researchers tried attaching white plastic to the nests of younger and older birds, and found that black kites who were not in top fighting shape quickly removed it.
"This could be similar to the use of different colors for the belts of judo or karate fighters," said Blas.
"You don't want to have a color that doesn't match with your fighting capability because you won't get any benefit from it."
Explore further: Too many chefs: Smaller groups exhibit more accurate decision-making