The effects of parental age on reproductive performance in the shag Phalacrocorax aristotelis

An improvement in reproductive performance over the first few breeding attempts is widespread among iteroparous breeders. A number of mechanisms have been suggested to explain this pattern, including age-related changes in competence and effort, improvements in average quality with age, and extrinsic effects. In this study, I aimed to investigate some of the mechanisms underlying age-specific changes in reproductive performance in the shag Phalacrocorax aristotelis. As well as having a lower breeding success, in most species young breeders also breed later in the season than older birds. Therefore, age-related improvements in breeding performance may be due to differences in environmental conditions experienced by young and older breeders, rather than intrinsic differences in breeding capacity. Experimental manipulations are therefore required to separate the confounding effects of age and timing of breeding on reproductive performance. In this study, hatching date was manipulated in young and older shags using a clutch cross-fostering protocol, such that both age classes reared their chicks at the same time, both early and late in the season, from eggs of a similar quality. Older pairs performed consistently better, rearing significantly more chicks both early and late in the season. Thus, the age-specific improvement in breeding success in the shag is not a result of extrinsic factors, but is due to intrinsic differences in brood-rearing capacity. A key hypothesis proposed to explain underlying intrinsic differences in breeding success is an age-related improvement in foraging performance. This theory proposes that young breeders are unable to meet the extra demands of rearing offspring because of their reduced foraging capabilities. The consequences in sexually size dimorphic species may be that young pairs will have particular problems rearing offspring of the more expensive sex. Most studies of sexually size dimorphic species have found that the larger sex is more expensive to rear in absolute terms. In the shag, the male is 20% heavier than the female, and sons and daughters fledge at a similar size disparity. When breeding in the same environmental conditions, and from eggs of the same quality, this study demonstrated that sons reared by young pairs grew more slowly and reached a lower peak mass than sons reared by older pairs, whereas there was no equivalent difference for daughters. This suggests a difference in young and older pairs' provisioning capacity, such that young pairs can provide sufficient food to daughters, but are not able to deliver the additional food required for sons. This has important implications for optimal age-specific sex ratios. The cross-fostering protocol provided two opportunities to compare the foraging behaviour of young and older pairs rearing their chicks at the same time, using radio telemetry. Each individual was followed for a single, complete feeding trip. Early in the season, birds were flying short distances, and making few dives. Late in the season, birds were flying greater distances, and making many more dives. This latter period was also characterised by many broods in the colony being left unattended, a previously undocumented occurrence in shags. Overall, therefore, there was strong circumstantial evidence that feeding conditions were considerably poorer late in the season. There were no differences with respect to age in mean dive duration or mean foraging depth either early or late in the season. However, at the end of the season, older birds had a significantly lower surface to dive duration ratio compared with younger breeders. Older birds were therefore spending less time resting between dives, thereby increasing the time spent foraging. There was no difference between the two age groups when foraging conditions were better early in the season. Thus, there was an interaction between age and environmental conditions on foraging behaviour. (Abstract shortened by ProQuest.).