SEX-RATIO VARIATION AMONG BROODS OF GREAT REED WARBLERS ACROCEPHALUS-ARUNDINACEUS

The sex of 746 great reed warbler fledglings (from 175 broods) was det ermined by the use of single primer polymerase-chain reaction. The rel iability of the technique was confirmed as 104 of the fledglings were subsequently recorded as adults of known sex. The overall sex ratio di d not differ from unity. Variation in sex ratios between broods was la rger than expected from a binomial distribution. Female identity expla ined some of the variation of brood sex ratio indicating that certain females consistently produced sex ratios that departed from the averag e value in the population. The theory of sex allocation predicts that parents should adjust the sex ratio of their brood to the relative val ue of sons and daughters and this may vary in relation to the quality of the parents or to the time of breeding. In the great reed warbler, the proportion of sons was not related to time of breeding, or to any of five female variables. Of five male variables, males with early arr ival date tended to produce more daughters. The sex ratio of fledgling s that were a result of extra-pair fertilizations did not differ from that of legitimate fledglings. Hence, there is currently no evidence o f that female great reed warblers invest in a higher proportion of son s when mated with attractive males.