Extreme gender-based post-fledging brood division in the toc-toc

The possibility that parents of one sex may preferentially invest in offspring of a certain sex raises profound evolutionary questions about the relative worth of sons and daughters to their mothers and fathers. Post-fledging brood division—in which each parent feeds a different subset of offspring—has been well documented in birds. However, a lack of empirical evidence that this may be based on offspring sex, combined with the theoretical difficulty of explaining such an interaction, has led researchers to consider a gender bias in post-fledging brood division highly unlikely. Here we show that in the toc-toc, Foudia sechellarum, post-fledging brood division is extreme and determined by sex; where brood composition allows, male parents exclusively provision male fledglings, whereas female parents provision female fledglings. This is the first study to provide unambiguous evidence, based on molecular sexing, that sex-biased post-fledging brood division can occur in birds. Male and female parents provisioned at the same rate and neither offspring nor parent survival appeared to be affected by the sex of the parent or offspring, respectively. The current hypotheses predicting advantages for brood division and preferential care for one specific type of offspring are discussed in the light of our results.