Data from: Limited evidence of biased offspring sex allocation in a cavity-nesting conspecific brood parasite

Sex allocation theory predicts that mothers should bias investment in
offspring toward the sex that yields higher fitness returns; one such bias
may be a skewed offspring-sex ratio. Sex allocation is well-studied in
birds with cooperative breeding systems, with theory on local resource
enhancement and production of helpers at the nest, but little theoretical
or empirical work has focused on birds with brood parasitic breeding
systems. Wood ducks (Aix sponsa) are conspecific brood parasites, and
rates of parasitism appear to increase with density. Because female wood
ducks show high natal philopatry and nest sites are often limiting, local
resource competition (LRC) theory predicts that females should overproduce
male offspring—the dispersing sex—when competition (density) is high.
However, the unique features of conspecific brood parasitism generate
alternative predictions from other sex allocation theories, which we
develop and test here. We experimentally manipulated the nesting density
of female wood ducks in four populations from 2013-2016 and analyzed the
resulting sex allocation of >2000 ducklings.  In contrast
to predictions we did not find overproduction of male offspring by females
in high-density populations, females in better condition, or parasitic
females; modest support for LRC was found in overproduction of only female
parasitic offspring with higher nest box availability. The lack of
evidence for sex ratio biases, as expected for LRC and some aspects of
brood parasitism, could reflect conflicting selection pressures from nest
competition and brood parasitism, or that mechanisms of adaptive sex ratio
bias are not possible.