Older and wiser: improvements in breeding success are linked to better foraging performance in European shags

1. An increase in average breeding performance with age and experience among younger age classes has been recorded in numerous studies of iteroparous breeders. An important component of this pattern is thought to be improvements in foraging performance, resulting in delivery of more or better quality food to offspring by older, more experienced individuals. 2. Young, inexperienced breeders may exhibit lower foraging efficiency or foraging effort, and it has been predicted that differences in foraging performance with age and experience will be more marked when environmental conditions are poor. However, as the timing of breeding generally differs with age and experience, intrinsic differences in foraging abilities are typically confounded by variation in extrinsic conditions, and hence food availability. 3. To disentangle these effects, we experimentally manipulated the timing of breeding in European shags, Phalacrocorax aristotelis Linnaeus. We used a cross-fostering protocol, such that naive, young birds reared their chicks at the same time as older, experienced individuals. Our design produced simultaneous chick rearing during two periods in the same breeding season that differed markedly in environmental conditions: early, when conditions were good; and late, when conditions were poorer. We examined foraging efficiency, foraging effort and amount of food delivered to offspring by the two classes of breeder. We predicted that any differences in foraging performance would be more marked under the poorer conditions later in the season. 4. We found that experienced parents delivered more food than naive parents, irrespective of the time of season. This was due partly to the consistently higher foraging efficiency of the experienced parents. In addition, experienced parents adjusted their foraging effort to the environmental conditions. Early in the breeding season, they made less foraging effort than naive parents. Under the poorer foraging conditions, experienced parents increased their foraging effort but naive parents did not, being either unable or unwilling to do so. 5. Our results suggest that an increase in foraging efficiency, and the capacity to adjust foraging effort in response to food availability, are important components of the observed improvements in breeding performance with age and experience.