Changes in body condition in breeding kittiwakes Rissa tridactyla

We investigated the seasonal pattern of changes in body condition of breeding black-legged kittiwakes Rissa tridactyla in Svalbard (79°N) to evaluate whether changes in body condition were a consequence of the energetic demands of breeding (the reproductive stress hypothesis) or of voluntary anorexia to attain lower flight costs during chick rearing (the programmed anorexia hypothesis). Adult body condition was recorded from early egg laying to fledging and was examined in relation to date (relative to hatching), sex, parental time-budget, brood size and reproductive success. To distinguish between the two hypotheses we evaluate whether the reduction in body condition occurred during or ahead of the energetically most demanding part of the chick-rearing period. We combine our results on changes in body condition and time-budget with published information on field metabolic rate (FMR) and chick energy requirements from studies in the same colony. Our calculations of adult energy requirements and energy intakes indicate that the first part of the chick-rearing period was energetically the most demanding period, because adult energy requirement per hour spent off the nest was highest in this period, and adults were time constrained because of the need for 24-h brooding of the chicks. During the incubation period female body condition increased slightly, but significantly, while male body condition was stable. During the first part of the chick-rearing period, female and male body condition decreased by 14.8% and 8.4%, respectively. During the second part of the chick-rearing period, both male and female body condition remained stable. The reductions in body condition occurred during the phase which was suggested to be the energetically most demanding part of the chick-rearing period, thus supporting the reproductive stress hypothesis. Parental body condition during the incubation period was positively related to the probability of successfully fledging young, providing additional support for the reproductive stress hypothesis.