Data from: Persistence of an extreme male-biased adult sex ratio in a natural population of polyandrous bird

In a number of insects, fishes and birds the conventional sex roles are reversed: males are the main care provider whereas females focus on matings. The reversal of typical sex roles is an evolutionary puzzle, because it challenges the foundations of sex roles, sexual selection and parental investment theory. Recent theoretical models predict that biased parental care may be a response to biased adult sex ratios (ASRs). However, estimating ASR is challenging in natural populations, because males and females often have different detectabilities. Here we use demographic modelling with field data from 2101 individuals, including 579 molecularly sexed offspring, to provide evidence that ASR is strongly male-biased in a polyandrous bird with male-biased care. The model predicts 6.1 times more adult males than females (ASR = 0.860, proportion of males) in the Kentish plover Charadrius alexandrinus. The extreme male-bias is consistent between years, and concordant with experimental results showing strongly biased mating opportunity toward females. Based on these results we conjecture that parental sex role reversal may occur in populations that exhibit extreme male-biased ASR.chick_sexData to estimate brood sex ratio at fledging (BSR, see equation S1 in Supporting Information). Variables: BROOD - brood ID, MALE - number of male chicks in the brood, FEMALE - number of female chicks in the brood, MAHATCH - hatching date (number of days since 1 March), AGE - brood age at capture. BSR was estimated at the middle of the four periods: 49, 72, 95, 117.5 days since 1 March.fledging_successData to estimate fledging success (FS, see equation S2 in Supporting Information). Variables: BROOD - brood ID, MAHATCH - hatching date (number of days since 1 March), FLED - whether the brood fledged any chicks (1) or not (0). FS was estimated at the middle of the four periods: 49, 72, 95, 117.5 days since 1 March.brood_sizeData to estimate number of broods hatched (B) and average brood size (BS) for the four periods. Variables: BROOD - brood ID, MAHATCH - hatching date (number of days since 1 March), BROODSIZE - number of chicks hatched, PERIOD - categorical variable giving the four periods