Effects of Parental Age and Environmental Change on Offspring Sex Ratio in a Precocial Bird

Abstract

1. Variation in offspring sex ratio at the end of parental care may reflect passive sexual differences in offspring survival, or a facultative manipulation by parents to balance trade-offs between sex-specific offspring growth and survival and parental fitness. In contrast, among species with limited size dimorphism, there may be little reason to expect significant sexual differences in offspring survival, and the presence of offspring sex-ratio variation in such species may indicate sex-specific differences in parental investment at some stage. However, sex-ratio variation in monomorphic species with praecocial young may be more difficult to explain, since the decreased parental investment, which is largely shared between offspring of either sex, may reduce the opportunity for sex-ratio manipulation. 2. We examined this question by analysing annual variation in fledging sex ratio of an avian herbivore with strong female natal philopatry and praecocial young, the lesser snow goose Anser caerulescens caerulescens, using data from a 23-year observational study during a period of marked population growth. Although snow geese show limited sexual size dimorphism, with males being 2-6% larger at all ages posthatching, growth and survival of snow goose goslings has been shown previously to be extremely sensitive to variation in food supply. 3. The proportion of males at fledging has decreased from 51% in the early years, when food at the colony was abundant, to c. 48% in recent years, when food supply has been reduced. Concurrent with this change, the mass and condition of goslings have declined significantly over the years, with males showing a proportionately greater decline than females. Hatching sex-ratio data showed no systematic significant differences overall, and thus the most likely explanation for the change in sex ratio is increased mortality of male goslings between hatching and fledging. 4. The change in fledging sex ratio occurred only among broods from younger adults (age < 5 years). These age-specific differences were evident both among and within individual females. Younger adults may be less able to accommodate long-term changes in foraging conditions successfully, resulting in proportionately greater gosling mortality overall, and increasingly biased mortality of male goslings, which may be more susceptible to food restriction. 5. Despite potential differences in indirect costs of rearing male or female offspring, it is suggested that these results are more consistent with increased mortality of male goslings under increasing food restriction, rather than active parental manipulation.