Offspring sex ratio in the Common Tern Sterna hirundo, a species with negligible sexual size dimorphism

In many vertebrates, male offspring are affected more than female offspring by adverse conditions during growth, resulting in facultative adjustment of offspring sex ratio by parents in response to social and environmental conditions during breeding. The greater vulnerability of male offspring is generally attributed to their higher energy requirements associated with their larger size, although greater sensitivity to adverse conditions could be related to other factors such as negative effects of androgens on male physiology. To control for sexual differences in body size, we examined variation in offspring sex ratio in the Common Tern Sterna hirundo , a species with negligible sexual size dimorphism. In this species, the last-laid egg (termed the c-egg) is smaller than the first two and hatches last, so that the chick obtains relatively little food and hence has a low probability of survival to fledging. This species thus provides a powerful model for examining sex-linked mortality and sex ratio variation under natural conditions. We found that the sex ratio of c-eggs, but not of earlier laid eggs, was significantly biased in favour of females. Chicks hatched from c-eggs (termed c-chicks) had low survival but female c-chicks had significantly higher survival than male c-chicks. These data provide strong evidence that factors other than sexual size dimorphism are responsible for producing greater vulnerability of male offspring to adverse conditions during growth.