Testing the Mediating Role of Social Interactions on the Relationship Between Age and Plumage Traits on Reproductive Success

Abstract

Differential reproduction is a key driver of evolution that is determined by individual characteristics and mating opportunities, including mate choice. Social interactions between conspecifics are hypothesized to be important in facilitating mate choice and reproductive success but are difficult to measure. Using data from 52 adult barn swallows (Hirundo rustica erythrogaster), whose social interactions were measured via proximity tags, we tested the hypothesis that social interactions mediate both the relationship between age (a proxy for experience) as well as condition-dependent plumage traits and their associations with reproductive success. We found that older female barn swallows had higher fecundity and that older males have higher paternity. Older males achieved higher paternity through extra-pair copulations, not by greater paternity with their social mate. Longer tail streamers were associated with greater fecundity/paternity in both sexes, but this effect was independent of age only among females. Darker ventral plumage coloration was not associated with higher reproductive success in either sex. We observed that older males appear to be less social with conspecifics, as indicated by fewer numbers of social interactions, though these associations were only marginally significant. The size of the effect of female age on conspecific social strength was similar to males but was not significant in females. Interestingly, females with fewer social interactions had higher fecundity. Finally, we found no evidence of mediation by the number of social interactions. Taken together, our results suggest that older, more experienced birds can produce more offspring without being more social.