Relative contribution of euphausiid prey species and timing of breeding to Cassin's auklet Ptychoramphus aleuticus growth: within-season effects

Abstract

Within-season variation in life-history parameters is characteristic of long-lived seabirds breeding in dynamic marine environments. Zooplanktivorous Cassin's auklets Ptychoramphus aleuticus breeding in central California feed primarily on euphausiid crustaceans, and switch from provisioning their offspring with Euphausia pacifica early in the nestling period to Thysanoessa spinifera later in the nestling period. I examined the effects of seasonal variation in nestling diet composition and meal mass, and timing of breeding and breeding attempt number (only/first or second breeding attempt) on the growth rate of individual nestlings on Southeast Farallon Island, California. For individual years, there were variable seasonal trends in growth, but no trends were apparent when all years were combined. Along with the predicted seasonal shift in prey use, there was a concurrent decrease in meal mass. In multivariate models, there were significant and positive effects of the mass proportions of E. pacifica and T. spinifera in the diet, meal mass and breeding attempt number on nestling growth, while the effect of hatching date on growth was negative. The relative influence of T. spinifera on growth was stronger than that of E. pacifica, the effects of hatching date on growth were likely dependent on the effects of breeding attempt, and breeding attempt exerted the strongest effect on nestling growth of all parameters examined. These suggest that the seasonal switch in prey use is advantageous (whether intentional or otherwise). Results illustrate the importance of adequate availability of both E. pacifica and T. spinifera to auklets in the Gulf of the Farallones. This study represents a unique approach in the investigation of linkages between individual nestling growth rate and diet and breeding phenology parameters by focusing on a fine temporal scale.