Red breast contains robin's details
Red-breasted robins may be an essential part of Christmas, but their distinctive plumage has far greater significance for the birds themselves, a study has shown.
The red breast varies in size and colouration with age and sex, suggesting it may communicate information about its owner, scientists believe.
As long ago as the 1940s, British ornithologist David Lack carried out a study in which a violent hen robin attacked a headless stuffed specimen.
The experiment, published in the journal Ibis, revealed that it was the red breast that triggered her aggression.
Scientists conducting the new research looked in more detail at the role of the robin's breast, as well as the grey fringe which frames the red feathers.
Both male and female robins have red breasts and defend their territory strongly all year round.
The researchers expected that both sexes would have similarly sized red breasts and grey fringes.
But observations in a pine forest north of Barcelona, Spain, showed surprise variations.
The size of the red breast was found to increase from the first to second year of life in both males and females. However, older males had larger red badges than females after their second year.
Computer models were also used which took account of environmental brightness and the way robins see colour contrast. They showed that, from a robin's point of view, the grey fringe which increases with age highlighted the edge of the red breast.