The Roseate Tern, Sterna dougallii, is an endangered species in the Northwest Atlantic, where it has undergone transient reductions in population size over the past 120 years. This population has been slow to regain former size and range, perhaps in part due to the female-biased sex ratio, which results in female-female pairs, reducing the average productivity of the colony. The larger populations of the Western Pacific and Indian Oceans are not endangered and there is no evidence of a biased sex ratio at breeding in Western Australia. We developed four novel microsatellite markers and adapted one other and these are the first used in the genus Sterna. We also determined the utility of these markers for 17 related species. Here we report the population genetic structure within and between two regions, the Northwest Atlantic and Western Australia. A significant finding is that the Northwestern Atlantic region has much lower allelic diversity than the Western Australia region, promoting the recommendation for increased protection of sites in this region in order to preserve remaining genetic diversity and new potential breeding habitats.
Several hypotheses have been proposed to explain facultative manipulation of sex ratios in birds, but existing data are inconsistent within and among species, and do not clearly support any one of the hypotheses. The sex ratio among breeding Roseate Terns (Sterna dougallii) at Bird Island, Massachusetts is female-biased (56%). We sought to determine at what stage of the life cycle this sex ratio bias is established. We monitored 461 eggs from 252 nests and, using molecular markers, we sexed 342 chicks at hatching and followed them to a stage when survival (or non-survival) to fledging could be inferred. We found that the sex ratio at hatching (and, by inference, at fertilization) was biased toward females (55%). This bias was significant in chicks from first-laid eggs (58%) but not second-laid eggs (48%). We also found that the overall bias increased (to 56.6%) during the chick stage through differential loss of male chicks.
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