Winter associations predict social and extra-pair mating patterns in a wild songbird

Despite decades of research, our understanding of the underlying causes of
within-population variation in patterns of extra-pair paternity (EPP)
remains limited. Previous studies have shown that extra-pair mating
decisions are linked to both individual traits and ecological factors.
Here, we examine whether social associations among individuals prior to
breeding also shape mating patterns, specifically the occurrence of EPP,
in a small songbird, the blue tit. We test whether associations during the
non-breeding period predict (1) future social pairs, (2) breeding
proximity, i.e. the distance between breeding individuals, and (3) the
likelihood that individuals have extra-pair young together. Individuals
that were more strongly associated (those that foraged more often
together) during winter tended to nest closer together. This, by itself,
predicts EPP patterns, because most extra-pair sires are close neighbours.
However, even after controlling for spatial effects, female-male dyads
with stronger social associations prior to breeding were more likely to
have extra-pair young. Our findings reveal a carry-over from social
associations into future mating decisions. Quantifying the long-term
social environment of individuals and studying its dynamics is a promising
approach to enhance our understanding of the process of (extra-) pair
formation.