Individual variation in field metabolic rate of kittiwakes (Rissa tridactyla) during the chick-rearing period

Field metabolic rate (FMR), using the doubly labelled water (DLW) method, w as measured in free-ranging adult kittiwakes (Rissa tridactyla) early and l ate in the chick-rearing period at Svalbard, Norway. Individual variation i n FMR was analysed by comparing FMR with body mass, sex, nest attendance, c hick age, brood size, and basal metabolic rate (BMR). Mean FMR of kittiwake s during the chick-rearing period was 27.0+/-0.9 (SE) W kg(-1), while the i ndividual variation (calculated as coefficient of variation [CV]) in FMR wa s 24%. Sex, time spent away from the nest, age of the chicks, and brood siz e contributed significantly to FMR and explained 65% of the variation in FM R. The FMR increased by 32% from early until late in the chick-rearing peri od. This occurred simultaneously with an increase in the time spent away fr om the nest. In 15 of 20 pairs, one of the mates had 15% or higher (mean of the 15 pairs,) FMR (W kg(-1)) than their partner, even though 22%+/-8% the mates spent equal amounts of time away from the nest. This indicates an in trapair conflict in FMR. The variation in total FMR of pairs was 40% less t han the individual variation, and total FMR of pairs increased with age of the chicks. This indicates that the mates adjust their energy expenditure w ithin a relatively constant FMR determined by the energy needs of the chick s. Individual variation in FMR could not be explained by variation in body mass or BMR. BMR measured late in the chick-rearing period was 26% lower th an previous measurements of BMR from the prebreeding and incubation periods . The increase in FMR and simultaneous decrease in BMR caused a 40% increas e in metabolic intensity (FMR/BMR) of kittiwakes during the chick-rearing p eriod. It is suggested that the metabolic intensity is not a proper measure of the metabolic load in seabirds.