Son or daughter, it does not matter: brood parasites do not adjust offspring sex based on their own or host quality

Abstract

Sex allocation theory posits that parents should adjust offspring sex ratio based on the costs and benefits associated with the production of either sex in a given context. Maternal condition should influence sex ratios when it has sex‐specific impacts on offspring fitness (Trivers–Willard hypothesis) or when rearing costs differ between sons and daughters (cost of reproduction hypothesis). In sexually size-dimorphic species, mothers in good condition are predicted to produce an excess of offspring of the larger sex, whereas mothers in poor condition the opposite. Brood parasites constitute an exciting model for testing sex allocation theory as parasitic females are freed from rearing costs while these costs are covered by the hosts. Here, we investigate the effect of maternal and host quality (both expressed as egg volume and blue-green chroma) on offspring sex allocation in the common cuckoo (Cuculus canorus) parasitizing the great reed warbler (Acrocephalus arundinaceus). We found no significant effect of parasite egg-laying date and maternal or host quality on sex ratio. One explanation may be that parasitic females, instead of investing in differential sex allocation, invest in securing egg acceptance by the host, by laying non-randomly within a host population to match the appearance of host clutches. Alternatively, male and female parasite eggs do not differ in size suggesting that their production bears comparable costs. This, together with the fact that the whole parental care is covered by the hosts, supports the previous findings that the common cuckoo does not adjust offspring sex ratio.