Parental provisioning behaviour predicts survival of Cape Gannet chicks under poor conditions

Cape Gannets Morus capensis are declining in numbers in all breeding colonies at the southern African west coast, potentially caused by deteriorating food availability. The behavioural responses of individuals to changing conditions can provide insights into the mechanisms that drive population dynamics. We studied the relationship between parental provisioning behaviour and the breeding success of Cape Gannets breeding at Ichaboe Island (Namibia) in two years with different nesting success, to assess the potential role of variation in food availability in explaining variation in chick survival. In 2006/07 parents delayed the onset of breeding and visited nests at a lower frequency than in 2005/06, indicating that food availability was limited. Within the years individual differences in provisioning rate but not in parental condition were associated with chick growth and survival. These associations remained present when controlling for seasonal changes. Predation, not starvation, was the main cause of mortality. Under poor conditions, as indicated by long foraging trips, Cape Gannet parents left small chicks unattended, resulting in many being killed by Kelp Gulls Larus dominicanus. Long foraging trips were associated with reduced average breeding success, but impacted some parents more than others. Knowledge of the origin of this inter-individual variability is needed to predict how changes in food availability affect seabird population dynamics, which is one of the steps towards conservation of these birds.