Energy Requirements of Seabird Communities: A Bioenergetics Model

Abstract

SUMMARY (1) Methods used to estimate energy requirements of avian communities are listed, and their inadequacies discussed. In particular, the published estimates lack confidence intervals. (2) A simulation model is described which estimates daily energy requirements of seabird populations. Biomasses of breeders, non-breeders and chicks present in the vicinity of the colony, are coupled with an ambient temperature function, the bioenergetic equations of Kendeigh, and calculations of the energy costs of digestion, activity, egg production and growth. (3) The model is tested using input data sets for great skua Catharacta skua Brinnich and Arctic tern Sterna paradisaea Pontopp. Sensitivity of the model to errors in input parameters, and the output errors resulting from known imprecisions in the forty-four input parameters are examined. Output precision is largely determined by a few parameters, which depend partly on the life-style energy allocation of the species. (4) A Monte Carlo simulation is used to determine the precision of the estimated energy requirements of the two species' populations. This suggests that, using currently available input data, the population energy requirement estimate has a 95% confidence interval of + 50o of the mean. (5) Energy requirements could be considerably more precisely determined if only a small number of input parameters were more precisely known. These limiting parameters are listed. (6) The model is applied to one particular seabird community, on Foula, Shetland, for which detailed data have been collected. The mean estimate of the total energy requirement of this community (1 2 x 101 0 kcal yr - 1) represents 29% of the mean estimate of the total annual fish production within a 45 km radius of the colony. This implies that seabirds, predatory demersal fish and industrial fisheries are in direct competition for the energy flow from pelagic fish, such that any increase in the energy flow to one of these would result in a reduced energy availability to the others.