Nestling sex ratios in a population of Bluethroats Luscinia svecica inferred from AFLP (TM) analysis

We studied the sex ratio of Bluethroat Luscinia svecica broods using AFLPs. Our aim was to test whether there is a bias towards males that could be ex plained by sexual selection thee-tries, or conversely, a bias towards femal es that could help explain the female-biased sex ratio among juveniles obse rved at a wintering site. The AFLP technique was reliable in sexing the nes tlings from even small initial DNA quantities. Given the large number of po lymorphic markers that can be obtained for each primer combination, the pro bability of detecting a W-chromosome-linked fragment is reasonably high. As a consequence, this method could be used in other species far sex-ratio st udies and for other genetic purposes. Among 246 nestlings. we found an over all proportion of males of 50.8% at hatching and the sex-ratio variation us ing broods as independent units was not significantly different from expect ation under a binomial distribution. None of the parental and environmental variables tested changed significantly the deviance to the model. Thus, se x determination in the Bluethroat seems to match the classical Mendelian mo del of a 1:1 sex ratio and cannot explain the biased sex ratio towards juve nile females found at the wintering site.