Seasonal shifts in the provisioning behavior of chinstrap penguins, Pygoscelis antarctica.

Whether parents are able to adapt food gathering to rising offspring demands, or if they are controlled largely by extrinsic factors, is important for understanding key limits on fitness. Over seven breeding seasons, we studied the provisioning behavior of chinstrap penguins, Pygoscelis antarctica, at Seal Island, Antarctica, during parents' transition to leave broods of one or two chicks unguarded. By measuring the frequency, duration, and diel timing of foraging trips and the quantity of prey brought to chicks, we examined the extent to which variation in parents' feeding behavior could be attributed to provisioning costs which increase with chick growth. Estimates of the energy content of food loads were combined with foraging patterns and brood requirements to model parents' seasonal provisioning budget. The frequency of foraging trips increased from the guard to post-guard phase and was higher, and the seasonal effect larger, in parents with two chicks. The duration of overnight trips (16 h) increased with seasonally increasing night length; diurnal trip duration (8 h) showed no seasonal pattern. Birds exhibited a seasonal shift to diurnal foraging, a trend that was generally weaker in parents of smaller broods. Food loads increased with chick mass only in parents of one chick; parents of two chicks had larger but more constant food loads. Based on per trip calculations, parents foraging overnight could not have delivered to two-chick broods enough food to meet their demands unless chicks were small. Diurnal foragers (regardless of brood size) and overnight foragers with one chick could meet brood demands at chicks' peak mass. The combined daily effort of parents indicated that mated pairs on average had ample resources to meet chick demands through most of rearing. A brief period when demands could not be met was predicted in two-chick broods just before chicks were left unguarded and again as they neared fledging. Our findings suggest that penguins both increased provisioning frequency and favored foraging under higher light intensity in conjunction with increasing chick demands, tactics which required parents to leave chicks unattended. The ability to maintain intrinsic control over provisioning has bearing on how penguins may be limited by extrinsic constraints. Prey surveys conducted annually near colonies show abundant resources 10-20 km offshore with no consistent seasonal shifts in abundance. These findings support a prominent role for intrinsic factors in the foraging decisions of chinstrap penguins.