Parents should invest more in one sex of offspring if the fitness retu
rn per unit investment is higher for that sex. Sex-biased provisioning
may occur when sons and daughters differ in their needs or when there
is local resource competition or enhancement. We used feeding observa
tions and sex-ratio manipulations to determine if sex-biased provision
ing occurs in western bluebirds (Sialia mexicana). The sex ratio of th
e brood had no effect on feeding rates by adults at unmanipulated nest
s over a 6-year period. Adult males and females also did not differ in
the number of feedings made to sons and daughters in 13 videotaped ne
sts. Likewise, adult feeding rates to nests experimentally biased towa
rd sons or daughters did not differ significantly. Nestlings that were
closet to the nest hole and that reached highest were the most likely
to be fed. Sons and daughters did not differ in the begging behaviors
most likely to result in a feeding. We conclude that sex-biased provi
sioning does not occur in this population of western bluebirds and tha
t nestling behavior may be a more important determinant of feeding.