Data from: Are there indirect fitness benefits of female extra-pair reproduction? Lifetime reproductive success of within-pair and extra-pair offspring

The forces driving extra-pair reproduction by socially monogamous females,
and the resulting genetic polyandry, remain unclear. A testable prediction
of the hypothesis that extra-pair reproduction partly reflects indirect
selection on females is that extra-pair young (EPY) will be fitter than
their within-pair young (WPY) maternal half-siblings. This prediction has
not been comprehensively tested in a wild population, requiring data on
the lifetime reproductive success (LRS) of maternal half-sib EPY and WPY.
We used 17 years of genetic parentage data from song sparrows, Melospiza
melodia, to compare the LRS of hatched EPY and WPY maternal half-siblings
measured as their lifetime number of hatched offspring, recruited
offspring and hatched grandoffspring. EPY hatchlings were not
significantly fitter than WPY hatchlings for any of three measures of LRS.
Furthermore, opposite to prediction, EPY hatchlings tended to have lower
LRS than their maternal half-sibling WPY hatchlings on average. EPY also
tended to be less likely to survive to hatch than their maternal
half-sibling WPY. Taken together, these results fail to support one key
hypothesis explaining the evolution of genetic polyandry by socially
monogamous females and suggest there may be weak indirect selection
against female extra-pair reproduction in song sparrows.