Energy costs of chick rearing in Black-legged Kittiwakes (Rissa tridactyla)

We studied energy expenditure in adult Black-legged Kittiwakes (Rissa trido ctyla) with doubly labeled water to measure energy costs of chick rearing. We removed eggs from randomly selected nests and compared energy expenditur e late in the chick-rearing period between adults raising chicks and adults whose eggs had been removed. Adults raising chicks expended energy at a ra te 21% higher than adults from manipulated nests, apparently owing to diffe rences in activity patterns while away from the colony. No sex-specific dif ferences were detected in energy costs of chick rearing or energy expenditu re, although statistical power for these analyses was fairly low. Among the unmanipulated group, energy expenditure tended to be positively related to natural brood size. An ancillary goal of our study was to test hypotheses that describe how population-level field metabolic rates (FMRs) vary during chick rearing. We compared FMRs among kittiwakes raising chicks at a colon y in Alaska (61%09N) with those reported for a colony in Norway (76%30N). F MRs of adults raising chicks were nearly identical at the two colonies, sug gesting that adults may have preferred levels of energy expenditure during chick rearing that are relatively invariant with environmental conditions, and that are not adjusted according to adult survival probabilities.