Body mass, volume, and buoyancy of some aquatic birds, and their relation to locomotor strategies

Body mass, volume, and buoyancy of diving ducks, surface-feeding ducks, auklets, cormorants, and gulls were measured by means of water displacement. Relations of volume and buoyancy to body mass in surface-feeding ducks were linear and did not differ from those of diving ducks if scoters and eiders were excluded. These large-bodied sea ducks (scoters and eiders) had buoyancies 22–28% above values predicted by the common linear regression for surface-feeding ducks and smaller diving ducks. Thus, scoters and eiders have larger respiratory or plumage air volumes relative to body mass. Although relations of volume and buoyancy to body mass were similar among most ducks, volumes and buoyancies of gulls were much higher, and those of cormorants much lower, than those of ducks. Volumes and buoyancies of auklets were the same as those of ducks of similar mass. The significance to locomotion of the buoyancy of restrained birds depends on predive expiration and ptilosuppression, and reduced buoyancy from compression of air spaces at foraging depths typical for the species.