Sex-specific fledgling success and brood sex ratio in the Great Reed Warbler (Acrocephalus arundinaceus)

We examine sex ratio variation and sex-specific probability of successful fledgling in relation to hatching date across 376 broods of Great Reed Warblers (Acrocephalus arundinaceus). The sex ratio of complete broods as well as broods with partial mortality did not deviate significantly from parity (0.5 and 0.53, respectively). Variation in sex ratio between broods was not larger than expected from binomial distribution, thus females seem not to manipulate the sex ratio of their broods in the studied population. As a consequence, sex ratio did not vary in relation to hatching date, years and fishponds. Female offspring showed lower fledgling success than their brothers, but the relationship between probability of successful fledgling and hatching date differed between sexes. Fledgling success of female offspring declined with hatching date more strongly than the success of male offspring. Thus, our study shows that Great Reed Warblers do not adjust offspring sex to match observed seasonal sex-specific variation in survival.