Social pairing in the absence of reproductive senescence in a socially monogamous songbird

Abstract Reproductive senescence may affect mating decisions if older individuals produce fewer and lower-quality offspring, but the effect of reproductive senescence on mate choice decisions remains poorly understood. Mountain chickadees (Poecile gambeli) do not exhibit clear reproductive senescence within their natural lifespans and therefore offer a unique opportunity to test whether the absence or delay of reproductive senescence affects social mate choice for mates of certain ages. Given the lack of obvious reproductive senescence in this species, we predicted that older individuals would be preferred as social mates because pairing with an experienced mate is often associated with direct and indirect benefits. In apparent support of our predictions, we observed positive assortment for age within breeding pairs and older individuals were more likely to be paired with one another than expected by chance. However, assortative pairing also occurred within most younger age classes, and when we restricted our analyses to consider only new pairings, rates of assortative pairing within age classes were similar to those expected by chance. Thus, the observed pattern of age-related assortative mating in this system likely occurs due to longevity of some pair bonds rather than active assortment for age among individuals forming new pair bonds. Combined, our results suggest that choice of a social mate based on age may be constrained in socially monogamous mating systems when many individuals maintain long-term pair bonds.