Costs of rearing and sex-ratio variation

Submitted by avianbiology on 5 September 2014.

The broods of most avian species are composed of about equal numbers of males and females. However, under certain circumstances the brood sex ratio has been found to deviate from unity and as such situations are of special interest for evolutionary ecologists, these examples have attracted a lot of interest. In a recent, very interesting paper published in J. Avian Biol., Moreno-Rueda et al. report such on an example in the southern grey shrike. In this species, females are slightly larger than males and as they require more food during the nestling phase, they suffer higher starvation mortality during situations of food limitation. Most interestingly, the egg laying female respond to this by producing a brood biased towards males when food shortage can be anticipated, e.g. during cold and rainy breeding seasons or when mated to males of low quality. Thus, they invest more in the less costly sex when there is a risk of environmentally induced brood reduction.

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