The age at first reproduction can significantly impact fitness. We investigated the possible source of variation in the age at first reproduction (“sexual maturity”) and its consequences for lifetime reproductive success in a wild population of barn owls. This raptor is sexually dimorphic for two melanin-based plumage traits shown to covary with sex-specific behaviour and physiology. We observed that females were sexually mature earlier than males, an effect that depended on the colour of their plumage and birth date. Among females born early in the season, dark melanic (i.e. more pheomelanic with large and many black feather spots) yearlings were sexually mature earlier than light melanic females. The relationship was in the opposite direction in those born late in the season. In yearling males, the opposite result, albeit less pronounced, was discovered, i.e. lightly melanic males born early in the season were sexually mature earlier than dark melanic males, an effect that was in the opposite direction in males that were born late in the season. Individuals that matured faster produced a larger number of fledglings per year than individuals that matured slower, an effect that was found only in dark melanic females and in light melanic males. Dark melanic females also achieved a higher lifetime reproductive success (LRS) than light melanic conspecifics. Our results suggest that a light melanic plumage is beneficial in males and a dark melanic plumage in females suggesting sexually antagonist selection.

The study was conducted between 1991 and 2019 in western Switzerland on a population of barn owls (Tyto alba). This dataset contains information on six hundred and fifteen adults (319 males, 296 females) captured as breeders and ringed as nestlings, and hence their birth date was known with precision. It contains information on hatching date and laying date of the very first clutch for males and females, and on three melanic traits (i.e. pheomelanin colour score, size and number of eumelanic spots displayed on plumage) observed in this species.

The dataset contains three files. The "AllData.xlsx" file contains all the data we collected for individuals that were ringed as nestlings and that were further captured as breeders. The "First Year Breed.xlsx" file contains the data for all the individuals who reproduced during their first year of life. Finally, the "Lifetime Reproductive Success.xlsx" file contains all the information regarding LRS. In each file, there is a sheet that describes the different variables.