Badge, bill colour of house sparrow signal testosterone levels

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Washington, June 6 (ANI): A team of scientists has found that the colour of a house sparrow's badge and bill is indicative of its testosterone - and hence - its dominance and quality.

According to scientists at Max Planck Institute for Ornithology in Seewiesen, Germany, the size of the badge was independent of hormone levels and the levels are generally higher before and during breeding season.

"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 edges are worn off and the black bib is clearly visible", says Silke Laucht who performed the study.

The badge and bill colour establish dominance ranks over other sparrows. 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.

Drawbacks of high testosterone levels, however, are weak immune system and stress resistance. Also, at no time during the year was there a correlation between the size of the badge and blood testosterone levels.

"Other studies have found correlations between badge size and age and body size of the animals", said Laucht.

The detailed study revealed another surprise: males with the highest testosterone levels during the breeding season did not have inevitably the highest levels during moult. How can ornaments that are developed during moult honestly signal their information many months later during the breeding season? For Silke Laucht and her co-authors this is a contradiction that they want to solve next.

The research was published online in Behavioural Ecology and Sociobiology, May 29th 2010. (ANI)

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