Trade-offs in prey quality and quantity revealed through the behavioral compensation of breeding seabirds

Abstract

Many productive ocean ecosystems are also highly variable, resulting in complex trophic interactions. We analyzed interannual patterns in the diet of a seabird, the common murre Uria aalge, in a region of high oceanographic productivity, the northern California Current, to investigate how these top predators adjust their chick provisioning to cope with environmental variability. Murres relied chiefly on Pacific herring Clupea harengus pallasi and surf smelt Hypomesus pretiosus to provision chicks, although they regularly returned 8 other fish taxa. Pro- visioning success was measured by the energy return rate to chicks, which in turn was disarticu- lated into energy per meal (quality) and meal delivery rate (quantity). Parents exhibited 'compen- sation' during 2 years in which smaller, low quality prey were returned more quickly than in years with normal (i.e. 'good') provisioning. Despite the increased delivery rate, energy return rates were still lower in 'compensation' vs. 'good' years. The lowest energy return rates occurred in 3 'poor' years, during which ocean productivity was also depressed. Our results suggest that mur- res in this system have the ability to shift provisioning strategies to deal with some variability in prey resources, but not when limited by exceptionally poor environmental conditions.