THE CONTRIBUTIONS OF BODY-TISSUES, RESPIRATORY SYSTEM, AND PLUMAGE TOBUOYANCY IN WATERFOWL

This study examines buoyancy partitioning between body compartments in diving and surface-feeding Anatidae. Body tissue and plumage densitie s and the volumes of air in the respiratory system and plumage were an alyzed using cadavers and restrained live specimens of waterfowl. The densities of the skeleton and remiges of surface feeders were signific antly lower than those of divers but the differences were not sufficie nt to significantly reduce the body tissue density and buoyant force o f divers. There were no other statistically significant differences in the body tissue densities of diving and surface-feeding waterfowl. Th e densities of contour feathers, down, and remiges were markedly diffe rent from each other but the net buoyant force of the feathers was les s than 5 % of that caused by the air trapped in the plumage layer. All cadavers were negatively buoyant in the absence of air in the respira tory system and plumage. Despite similar net buoyant forces, there wer e large differences in the volumes of air in the respiratory system an d plumage between restrained ducks and cadavers. These results indicat e that the use of cadavers or restrained ducks to determine net buoyan cy, and hence power output, during voluntary foraging behaviour is unr eliable.