DIVING BIRDS IN COLD WATER - DO ARCHIMEDES AND BOYLE DETERMINE ENERGETIC COSTS

The volume of air trapped in the feathers and the body density of 36 species of water bird were determined by water displacement experiments. Body density was higher and the volume of air trapped in plumage was lower in species that were most reliant on diving for foraging. Accordingly, we predict that habitually diving species have substantially reduced energy expenditure while underwater and correspondingly higher aerobic dive limits than infrequent divers. This agrees with field observations. Following Boyle's law, aerobic dive limits are predicted to increase with increasing dive depth due to a reduction in upthrust following volumetric reduction of feather-trapped air caused by hydrostatic pressure. It appears energetically more costly for diving birds to forage near the surface than at greater depths. Reduced plumage air to minimize underwater swimming costs, however, probably increases heat loss. The use of fat for insulation is not compatible with minimized flight costs. Frequent divers have higher flight costs than infrequent divers. We predict that the amount of air in the feathers and the amount of subcutaneous fat in aquatic birds are regulated in such a way as to minimize energy expenditure as a function of the temperature of the environment as well as diving and flying rhythms.