Population and Energetic Aspects of the Relationship Between Blackbrowed and Greyheaded Albatrosses and the Southern Ocean Marine Environment

Summary

The methods and results of the study of the tropho-dynamic relationships between 2 Diomedea albatrosses and the marine environment at South Georgia are described. They illustrate the technical and theoretical developments necessary to obtain certain empirical data essential for accurate assessments of the role of seabirds in marine ecosystems. Differences in breeding success during eight yr (consistent in D. chrysostoma, more variable in D. melanophris) are linked with important differences in breeding frequencies which affect the size and activities of populations at the breeding sites. Extensive dietary studies, based on sampling adults about to feed chicks, showed major inter-specific differences, resulting in chicks receiving meals of similar size and frequency but of different energy content. The frequency of chick feeding was determined initially by daily and 3-h weighing. Recently automatic equipment has recorded weights every 10 min, giving the frequency and size of meals and resulting digestive performances of the chicks. Experiments involving exchanging chicks between the two species were combined with new methods for analyzing growth curves. They showed that, while there was a species-specific genetic component to growth, the overall rate could be significantly modified by the nature of the diet. The slower growth rate of D. chrysostoma chicks, and the species’ diet, are probably important factors affecting breeding frequency. Adult feeding performance is being studied by devices recording simple activity budgets at sea. Preliminary results are described and projected work linking this with the automatic weighing equipment and with assessment of foraging energy costs is outlined.