The energy economy of the arctic-breeding Kittiwake (Rissa tridactyla): a review.

The present paper reviews recent studies on changes in body mass, body composition and rates of energy expenditure during the breeding season in the black-legged Kittiwake (Rissa tridactyla) on Svalbard (79 degrees N). The main characteristic of the energy budget is a pronounced decrease in body mass as well as basal metabolic rate (BMR) after the eggs have hatched. While most internal organs lose mass in direct proportion to the general decrease in body mass, the liver and kidney masses decrease to a disproportionately greater extent. Since both the liver and the kidney have high intrinsic metabolic rates, these results support an earlier notion that the reduction in body mass is an adaptation to reduce maintenance costs. Alternatively, the reduced BMR is due to a decrease in energy uptake from the gastrointestinal tract, thereby ensuring that undigested food is ready to be regurgitated to the chicks. At the end of the chick-rearing period, the field metabolic rate (FMR) reaches its highest level, probably due to an increased workload associated with chick feeding. This occurs at a time of low body mass and BMR. A pronounced increase in the metabolic scope (FMR/BMR) during the latter part of the chick-rearing period demonstrates that BMR and FMR may change independently of each other and that the ratio FMR/BMR may not be a good measure of energy stress.