PATERNAL EXPENDITURE IS RELATED TO BROOD SEX-RATIO IN POLYGYNOUS GREAT REED WARBLERS

In many polygynous animals, parents invest more heavily in individual sons than in daughters. However, it is unclear if these differences in investment are a consequence of sex differences in the demand of offs pring related to sexual size dimorphism or a consequence of parental m anipulation. Here, we report on parental food delivery frequency in re lation to brood size and brood sex ratio in a wild population of polyg ynous great reed warblers Acrocephalus arundinaceus. We used the polym orphic microsatellite loci on the Z chromosome to sex chicks. We found that paternal feeding frequency (times/h per nest) increased not with brood size, but with the proportion of males in the brood, although t he demand per nest was more closely related to brood size than to broo d sex ratio. Additionally, the increase in rate of paternal feeding fr equency in relation to the brood sex ratio was much higher than the in crease in rate of nestling food demands. Maternal feeding frequency wa s independent of both brood size and brood sex ratio. These results st rongly suggest that fathers preferentially invest in their sons. We pr opose that parents can afford sex-biased parental care in animals in w hich food provisioning is enough for all offspring to survive.