Summary
Fledgling sex ratios biased in favor of the less costly sex in sexually dimorphic birds have been used as evidence in support of Fisher's theory that parents should invest equally in sons and daughters. However, such biased sex ratios are also predicted as a nonadaptive consequence of a 1: 1 hatching sex ratio and greater vulnerability to starvation of nestlings of the larger sex. We used estimates of the relative costs of males and females to predict the minimum sex-ratio bias at fledging that could possibly allow equal investment by independence, for seven species of sexually dimorphic birds. Although reported values for three of the species were potentially consistent with the values predicted by Fisher's theory, the observed values for the four most dimorphic species were far too male biased for investment at independence to be equal. We suggest that male-biased mortality among nestlings in sexually dimorphic species is the cost males bear for being large as adults and is the nonadaptive cause of biased fledging sex ratios in these species.