Dissecting direct and indirect parental effects on reproduction in a wild bird of prey: dad affects when but not how much

Males can through their behavior (e.g., courtship feeding) exert an indirect effect on their partner's reproductive traits, such as the seasonal timing and size of her clutch. Evidence for such indirect (male) effect on reproduction is starting to accumulate. We quantify female and male effects on reproduction in the tawny owl Strix aluco using a hierarchical mixed model on data collected in 1978-2013. We find that differences between males explain 7 % of the phenotypic variance in laying date (females 5 %). In contrast, females have a clear (11 %) effect on clutch size, whereas males have no effect. Based on multivariate hierarchical modeling, we find an individual-level correlation between the male-specific effect on laying date and his body mass (but not his plumage color or wing length). Heavy males may be able to affect their partner's seasonal timing of laying because of an advantage in providing courtship feeding prior to reproduction. Our findings illustrate that males can be an important determinant of variation in reproduction and that multivariate mixed models present a general approach to pinpoint which individual characteristics could be associated with such indirect effects.