Short-term fluctuations in forage fish availability and the effect on prey selection and brood-rearing in the black-legged kittiwake Rissa tridactyla

Abstract

To better understand how fluctuations in prey abundance may impact seabird reproductive success, we studied short-term changes in prey populations and their effect on prey selection and brood-rearing in the black-legged kittiwake Rissa tridactyla, a predator of near-surface-schooling forage fishes, our fine-scale approach involved a weekly assessment of forage fish abundance and brood-rearing conditions during 4 consecutive years (1996 to 1999) at the Shoup Bay kittiwake colony in Prince William Sound, Alaska. We conducted forage fish surveys from a fixed-wing aircraft to determine weekly prey abundance throughout the known foraging range of breeding kittiwakes. Our results provide clear evidence that short-term fluctuations in prey availability are responsible for dramatic, within-season changes in the breeding conditions of black-legged kittiwakes. Adult kittiwakes often showed immediate response to changes in the prey base by altering prey selection; however, there were instances when kittiwakes selected prey species disproportionate to their availability (typically selecting for Pacific herring Clupea pallasi and against Pacific sand lance Ammodytes hexapterus). Changes in prey selection often resulted in striking differences in the amount of time required to obtain a load of food. The cascading effects of longer foraging trips was translated into reduced nestling growth and survival. Of the 3 components of energy provisioning to nestlings (meal delivery rate, meal size, and energy density), meal delivery rate had the strongest and most consistent positive effect on nestling growth and survival. Overall, these results demonstrate that complex foraging conditions limit the reproductive success of a central place-foraging species relying on an ephemeral food source. Moreover, we demonstrated that feeding conditions during the first 2 wk of brood-rearing were most critical for survival of the brood. Given the potential for such marked within-season variation in breeding conditions, it is critical that investigators adequately sample throughout the brood-rearing period, or, alternatively, select that portion that is germane to their study.