Sex-specific costs of rearing a nestling and its implications in the brood sex ratio of Magellanic penguins

In birds, possible explanations for a bias in brood sex ratio include differential cost of rearing nestlings of different sexes, and different parental fitness returns related to offspring sex. We studied brood sex ratio of Magellanic penguins, Spheniscus magellanicus , in Puerto Deseado for 3 years. Our objectives were to compare the growth curves and energetic costs of rearing nestlings of different sexes, and to evaluate the possible implications of environmental and parental condition in the establishment of a bias in the brood sex ratio. We also investigated the relationship between hatching order and sex, and its impact on brood survival. Asymptotic mass was 11.41% higher for males than for females. The energetic cost of feeding male nestlings was slightly higher than for feeding females, but the difference in energy requirements was only 2.6% of the total energy budget. During the 3 years, brood sex ratio was 0.53, and almost constant within years over the raising period, showing no sex allocation during feeding. Sea surface temperature, which is linked to higher prey abundance when colder, explained brood sex ratio at fledging. The sex ratio was male-biased during the coldest year. Parental body condition was not an important variable explaining brood sex ratio. There was no bias in nestling sex with respect to hatching order and nestling survival was not related to nestling sex. We conclude that, even though the cost of feeding male offspring is higher, it only involves a small fraction of total cost of raising nestlings and might not be responsible for an adaptive bias in the sex ratio of nestlings for this species. However, during good oceanic conditions, females might bias their brood sex ratio towards males, thereby potentially gaining an advantage by raising good-quality males.