Spatiotemporal and genetic contingency of extrapair behaviour in a songbird

Multiple mating to obtain genetic benefits has been championed as the most likely cause of the evolution of polygamy. However, this assumption has been put into question by an increasing number of recent studies, most of which highlight the importance of considering ecological constraints to comprehend variation in extrapair (EP) behaviour. Here, we studied patterns of extrapair paternity (EPP) in the great tit, Parus major, using data from 11 nestbox plots that differed in population size and breeding conditions. Specifically, we analysed EPP rates in relation to socioecological variables that could influence the way individuals encountered one another in space and time, we tested whether adults engaged in EPP with more heterozygous, more compatible or phenotypically superior individuals than their social mates and we analysed whether extrapair offspring (EPO) were phenotypically or genetically superior to within-pair offspring. Our results do not provide support for the genetic benefit hypothesis from either the male or the female perspective. EPO were heavier than their within-pair paternal half-siblings, but there was no significant difference between EPO and their within-pair maternal half-siblings in terms of phenotypic quality. Regarding socioecological factors, we found a negative relationship between breeding synchrony and EPP rates both within and among plots, which suggests that males face a trade-off between mate guarding and obtaining EPP elsewhere. Our results show that most males engaged in EPP after the fertile period of their social female despite having to travel long distances: about half of the detected cases of EPP involved individuals from different woodlands. This study indicates that when and where to engage in EPP seem to be more relevant factors than with whom to do it and highlights the importance of considering spatiotemporal constraints at a landscape scale to achieve a better understanding of variation in EP mating behaviour.