Sons and daughters: age‐specific differences in parental rearing capacities

1. Numerous studies of iteroparous breeders have demonstrated an increase in average breeding performance with parental age. 2. One of the most widely suggested mechanisms to explain this pattern is that the foraging performance of young breeders is relatively poor. Where this is coupled to differences in the costs of rearing male and female offspring, for example because of sexual size dimorphism, young breeders may have difficulties rearing the more expensive sex. 3. This was investigated in the Shag, a monogamous seabird in which adult males are on average 20% heavier than adult females. Young pairs breed later in the season than older pairs, and lay smaller eggs. Differences in timing of breeding and egg quality were controlled for in a cross‐fostering experiment in which older and young individuals reared chicks simultaneously from eggs of similar quality. 4. Male chicks reared by young parents grew more slowly and attained a lower peak mass than those reared by older parents, whereas there was no equivalent difference for female chicks. 5. These results suggest that sons are energetically more demanding on their parents than daughters, and the more expensive sex fledges in significantly poorer condition when the parents are young. Therefore, it is predicted that optimal offspring sex ratio will vary with parental age.