Ornithologists studying the feathers of that Christmas card fixture, the Robin, say that the distinctive red breast may serve to communicate the unique features of each bird.
Findings just published in IBIS reveal that the red breast varies in size and colouration according to age and sex. The research, led by Dr Roger Jovani from Estación Biológica de Doñana (Spanish National Research Council, CSIC) in Sevilla, follows in the footsteps of ornithologist David Lack, who studied Robins in the 1940s. He recorded that a violent hen Robin attacking a headless stuffed specimen, suggesting that the red breast triggered the aggression.
“After more than six decades, we still have a very limited understanding of the important information Robins may communicate through the red breast,” said Dr Jovani. “Our study sought to understand how its size differs according to age and sex. Also, we studied the role of the grey fringe that frames the Robin’s red breast, which has been largely overlooked.”
The research, carried out at a pine forest north of Barcelona, focused on the sexual dimorphism and trends in red breast size when Robins age. Juvenile robins develop the read breast during their first summer moult.
The Robin moults again in its second summer and this, the team predicted, leads to a larger red breast. As both male and female Robins defend territory all year round the team expected that both sexes would have similarly sized red breasts and grey fringes.
The team’s results revealed unknown patterns of variation in the red breast and grey fringe of the Robin. The size of the red breast was found to increase from the first to the second year in both males and females. However, old males showed consistently larger badges than old females after their second year.
This surprising variation across sexes and over time suggests that the red breast may convey information of sex, age or related condition-dependent traits.
They also found that the grey fringe, which increases in age, highlights the perimeter of the red breast.
“Our study is a first step in understanding the role of the red breast and the grey frame in the life of robins,” said Dr Jovani. “Further studies will likely show more surprises on the role of the red breast and its grey frame on resolving territorial contests and mating decisions in robins.”
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