Individual and environmental determinants of reproductive success in male tree swallow (Tachycineta bicolor)

Evaluating the contribution of individual and environmental determinants of reproductive success is essential to improve our understanding of sexual selection. In socially monogamous bird species with high rates of extrapair paternity, traits or environmental contexts affecting the number of within-pair young (WPY) produced by males can differ from those affecting the number of extrapair young fathered (EPY). Here, we use a 4-year dataset collected in contrasted environments to assess the factors affecting male reproductive success in tree swallows (Tachycineta bicolor), a species showing high levels of extrapair paternity. Our analyses revealed that the number of WPY was higher under better environmental conditions, while the number of EPY was mainly related to male characteristics. Males nesting in more intensive agricultural areas had fewer WPY produced and a lower reproductive success. Also, males breeding earlier in the season had more WPY. The presence of parasites reduced males' reproductive success, mainly by reducing the number of EPY. The influence of male phenotype varied according to population density: Tarsus length variation had a greater effect on reproductive success at low population density than at high density, while wing length was also positively related to the number of EPY, more so at high than at low density. Altogether, our results suggest a complex interplay between individual and environmental determinants of reproductive success and imply that sexual selection dynamics varies depending on environmental contexts.