Data from: Maternal investment is positively associated with the presence of extra-pair offspring in a socially monogamous songbird

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.