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

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.