Population sex ratio variation in a monogamous long-lived bird, the wandering albatross

1. Factors affecting population birth sex ratio and variations of sex ratios within population in very long-lived animals are poorly known, apart for humans that have attracted much attention. We measured the age and sex structure of a population of a monogamous long-lived seabird, the wandering albatross Diomedea exulans, where reproductive performance of individuals is known, to examine factors affecting sex ratio variation and survival consequences of producing a particular sex. 2. The overall sex ratio at hatching is significantly biased toward the production of males. Very young and old parents tend to produce an excess of female offsprings, whereas intermediate-aged birds, which represent the bulk of the population, produce an excess of males. Low-quality parents (quality is measured as average breeding success over the cumulated past breeding life, and is not related to condition or age) produce more female chicks compared with high-quality parents that represent the bulk of the population. The combined effect of age and quality of parents produce an extensive sex ratio variation at the level of the population. 3. The survival of male and female parents is influenced differently by their quality and by the sex of offspring. Male parents die in larger numbers when rearing a female chick, whereas low-quality female parents have a lower survival, especially when they rear a male offspring, the sex needing higher parental investment. The differences between males and females parents' survival are probably due their contrasted investment in parental care. 4. After fledging, during the 5-8-year period of immaturity, there is a much higher mortality of males that balances the surplus of males at hatching and fledging and results in a similar number of males and females at the time of recruitment. However, the overall operational sex ratio is male biased due to the slightly higher mortality of adult females that results in an excess of old widowed males: excluding these post-reproductive males leads to an unbiased operational sex ratio. 5. These results show that sex ratio at hatching varies extremely with the age and phenotypic quality of parents, and lead to a complex age and sex structure of the population in this strictly monogamous long-lived species.