PROVISIONING IN WESTERN BLUEBIRDS IS NOT RELATED TO OFFSPRING SEX

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.