Extrapair paternity in Mediterranean blue tits: socioecological factors and the opportunity for sexual selection

The frequency of extrapair paternity within populations has been hypothesized to be related to ecological and social factors, which in turn can determine the impact of extrapair paternity on the opportunity for sexual selection. Here, we use the blue tit Cyanistes caeruleus as study species to assess both issues. In particular, we analyze patterns of extrapair paternity in 12 nest-box plots that greatly vary in local population size, level of nest-box aggregation, and breeding density. We found a significant positive relationship between extrapair paternity rate and local population size. Within study plots, neither local breeding density nor synchrony had an effect on the occurrence of extrapair paternity. Most extrapair males engaged in extrapair copulations with neighbouring females, probably in order to avoid paternity losses. Individuals that travelled larger distances to gain extrapair paternity likely did so because the social females of most of them had not yet begun their fertile period and, thus, within-pair paternity was not at risk. Variance in male reproductive success was mostly produced by variance in within-pair success, which in turn was primarily influenced by mate quality. Extrapair success contributed substantially to variance in male reproductive success (26%), but its effect was smaller than expected. Bateman gradients showed positive slopes (βss) for both males and females. However, the lack of a positive covariance between within-pair and extrapair success suggests that the effect of extrapair paternity on the strength of sexual selection was limited. This fact can be explained by the spatial distribution of extrapair fertilizations, which points to the absence of directional female mating preferences in this study system and, thus, not leading to "big winners" and "big losers."