# Maternal Investment Is Positively Associated With the Presence of Extra‐Pair Offspring in a Socially Monogamous Songbird

## TL;DR

Female songbirds invest more in their nests when they have offspring from outside their social pair, while males do not adjust their care.

## Contribution

This study shows that female investment increases with extra-pair offspring presence, a novel insight into avian parental behavior.

## Key findings

- Maternal visitation rates were higher in nests with extra-pair young compared to nests without.
- There was no relationship between paternal investment and the presence of extra-pair young.
- Females who participated in extra-pair copulations invested more in the resulting offspring.

## Abstract

Biparental care is common in socially monogamous avian species, but both partners may seek extra‐pair copulations (EPCs). The relative costs and benefits of EPCs between the sexes are likely complex, yet the implications of EPCs for parental care behavior have been examined predominantly in males. Not only could females benefit from EPCs, but females would have additional information about the likelihood of extra‐pair young (EPY) in their nest not available to their partners, which likely influences female behavior. We examined how the presence and abundance of EPY in a nest affect parental behavior in a socially monogamous songbird, song sparrows (
Melospiza melodia
). We predicted that females who mated outside the social pair would invest more in a clutch with a higher probability of EPY. We monitored nest visitation rates by male and female social partners as a proxy for parental investment and quantified extra‐pair paternity in 45 nests. Maternal visitation rates were higher in nests with EPY compared to nests without, while males did not adjust their investment in relation to the presence of EPY. These findings support our prediction that females who participated in EPC would invest more in the resulting offspring.

We explored the relationship between maternal and paternal investment (nest visitation rates) and the paternity of nestlings in a wild, socially monogamous songbird. We found that maternal investment was positively associated with the presence, number, and proportion of extra‐pair nestlings, but there was no relationship between paternal investment and extra‐pair young. These findings support our prediction that females who participated in extra‐pair copulations would invest more in the resulting offspring.