Abstract
Bill colouration is a highly variable trait with the potential to rapidly change over time. We observed strong interindividual and seasonal variation in the bill colour of captive female House Sparrows ( Passer domesticus ) and describe two colour traits by using a simple categorization: i) the presence or absence of yellow markings on the upper mandible and ii) whether the female bill is light horn-coloured or dark brown. In spring, females display either light horn-coloured or dark, almost black, bills and half of the birds have yellow markings on the upper mandible. However, this changes markedly when the birds start breeding. In our observation, after the first brood, the proportion of individuals with yellow markings was significantly lower and in none of the females that raised offspring this trait was present. However, during the non-reproductive period in autumn, the yellow pigmentation of the upper mandible became visible in all individuals. Furthermore, the proportion of females with light bills increased from pre-breeding to the non-reproductive stage. The distinct differences in bill colouration together with the seasonal changes may indicate a potential signalling function and might serve in sexual selection.