The size of the black breast bib the badge - and bill colour of male House Sparrows change over the course of the year. Such ornaments usually signal quality and dominance of a male to his conspecifics and are correlated with his testosterone levels. These levels are generally higher before and during breeding season than for example during moult in autumn. A team of scientists at the Max Planck Institute for Ornithology in Seewiesen, Germany, recently demonstrated in a detailed study that only bill colour was correlated with the amount of testosterone in the blood. In contrast, the size of the badge was independent of hormone levels (Published online in Behavioural Ecology and Sociobiology, May 29th 2010).
Sparrows live in social groups. All year round males engage in aggressive interactions to establish dominance ranks. External traits such as badge and bill colour so called ornaments serve as signals for conspecifics. For example, the bigger the comb of a rooster the more dominant he is. The elaboration of many such sexually selected ornaments from all kinds of animals depends on the hormone testosterone, which also causes dominant and aggressive behaviour. Nevertheless, these ornaments have inescapable costs because high blood testosterone levels suppress the immune system and stress resistance, and could thus eventually be costly for the animal. Therefore, ornaments are also called honest signals; only high quality animals can afford these costs.
Scientists at the Max Planck Institute for Ornithology in Seewiesen, Germany, were not only interested in the relationship between the elaboration of ornaments of male House Sparrows and blood testosterone levels but also if and how these relationships change over the course of a year. "The comparison of different seasons is a very important approach: the size of the badge is determined during moult in autumn, but maybe only important during the breeding season when the white feather ed
|Contact: Silke Laucht|