Reproduction is affected by individual breeding experience but not pair longevity in a socially monogamous bird

In socially monogamous animals, maintaining stable mating pairs across years has been hypothesized to result in increased reproductive success. However, previous individual breeding experience may independently affect reproductive success, regardless of pair stability. We examined associations between pair composition based on previous breeding experience, pair longevity, and reproduction in socially monogamous mountain chickadees, Poecile gambeli, in the Sierra Nevada. There were no significant differences in any reproductive parameters between pairs of experienced breeders that had bred together in previous years and those that were newly formed. Pairs in which both partners were experienced had heavier nestlings than both first-year breeders and mixed experienced–inexperienced pairs. Experienced females started laying eggs earlier and laid larger clutches regardless of their social mate’s previous breeding experience. As all experienced birds were older than inexperienced breeders, it remains possible that observed differences between these groups were due to age rather than breeding experience. Overall, our data did not support the hypothesis that pair longevity drives reproductive investment.