Age of the breeders, but not territory quality, explains hatching sex ratio in booted eagles

Abstract

Natal sex‐ratio variation is a central concept in population demography. When males and females are equally costly to rear, the population sex ratio should be 1:1, but if they differ in cost, more of the cheaper sex should be produced. According the age dependent sex ratio hypothesis, younger breeders should produce more chicks of the cheaper sex. On the other hand, the environmental sex determination hypothesis suggests that parents in poor conditions (scarce food and low‐quality territories) tend to produce more of the cheaper sex. To test hypotheses on adaptive sex ratio, we studied the booted eagleAquila pennata, a raptor with marked sexual dimorphism, with females larger than males. We sexed 174 nestlings from 121 broods in 1996–2000 to study hatching and fledgling sex ratios. We did not find deviations of the 1:1 sex ratio at the population level during the length of the whole study period or on a yearly basis. Sex ratio was affected by age of the breeders, but not by territory quality (average productivity), quality of year, nearest nest distance or number of breeding pair in the population. Older breeders raised more females (i.e. the expensive sex), supporting the age dependent sex ratio hypothesis. In long‐lived species, older breeders tend to occupy better quality territories than younger ones, confounding some of the commonly used surrogates of territory quality, like frequency of occupation or year of first occupation; results obtained from observational studies should be interpreted with caution. We discuss alternative approaches and different scenarios that could provide new insight into the study of factors that influence sex ratio deviations in vertebrates.