Male’s age and plumage coloration predicts brood sex ratio in the Common Kingfisher (Alcedo atthis)

Plumage ornaments of birds, both pigment-based and structural, are considered age or condition-dependent, honestly signalling male quality, sexually selected traits, influencing the variation in breeding performance as well as adaptive sex allocation. In the present study, we examine the effect of age of males on the plumage structural colour in Common Kingfisher, and evaluate the breeding output (laying date, total number of young per breeding season, mean number of young per nest, total number of broods) and sex ratio in relation to two ornamental traits (forehead and rump structural colour). During the three years 2016–2018 in the Danube river system (south-western Slovakia), we collected data from 49 males, 102 broods and 645 nestlings. Our data demonstrate that structural colour is a condition-related and age-sensitive signal, while males with increased weight expressed less saturated blue forehead feathers, and old males displayed more saturated cyan rump feathers than young ones. Moreover, the brood sex ratio varies with male coloration in an age-dependent manner. The plumage colour of young males did not affect the brood sex ratio, whereas old males with increasing intensity of cyan rump had more sons than duller ones. Neither single ornamental trait nor age predicted breeding output of kingfisher males. Nevertheless, our results indicate that the blue structural plumage of kingfisher males may serve as an indicator of age and a certain kind of quality with a consequence on adaptive sex allocation in this species.