Covariation between bill colouration and fitness components in a nocturnal bird

Vividly coloured chromatic signals play a key role in social and sexual signalling in diurnal birds, but their role is considered negligible in favour of achromatic (i.e. white, pure grey and black colourations) signals in nocturnal species. Here we studied colour variation and potential signalling of the yellow bill – a trait functioning as sexual signal in diurnal raptors during the breeding season in females of a truly nocturnal raptor, the little owl Athene noctua. We found that yellow‐red chroma of the bill was highly variable between individuals and positively correlated with female fitness prospects (i.e. brood size at fledging). In addition, we found that females with brighter bills were larger in size and produced owlets with a higher mass at fledging. This study suggests that yellow bill colouration in female little owls may potentially play a role in sexual signalling and may constitute the first evidence of chromatic colour signalling in a nocturnal bird.