Primary sex ratio bias in an endangered cooperatively breeding bird, the black-eared miner, and its implications for conservation

The aim of our study was to investigate primary and adult sex ratios in the cooperatively breeding black-eared miner, Manorina melanotis. We used gene tic methods to determine the sex of all birds. Observations were made to qu antify differences in helping behaviour between the sexes. As in other mine rs, Manorina spp., non-breeding males provided most of the help in raising young. Male and female nestlings did not differ significantly in weight, su ggesting that both sexes are equally costly to produce. Like other miners, the adult sex ratio in black-eared miners is male-biased (64.4%). However, unlike its congeners, the black-eared miner's primary sex ratio was strongl y biased toward females (62.5%). This suggests that females suffer higher j uvenile mortality than males. Our study illustrates how understanding sex r atios is both of theoretical interest and relevant to biological conservati on. (C) 2001 Elsevier Science Ltd. All rights reserved.