Provisioning in western bluebirds is not related to offspring sex

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Abstract
Parents should invest more in one sex of offspring if the fitness return 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 observations and sex-ratio manipulations to determine if sex-biased provisioning occurs in western bluebirds (Sialia mexicana). The sex ratio of the brood had no effect on feeding rates by adults at unmanipulated nests 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 nests. Likewise, adult feeding rates to nests experimentally biased toward sons or daughters did not differ significantly. Nesdings that were closest 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 provisioning does not occur in this population of western bluebirds and diat nesding behavior may be a more important determinant of feeding.

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