Diving metabolism and thermoregulation in common and thick-billed murres

The diving and thermoregulatory metabolic rates of two species of diving seabird, common (Uria aalge) and thick-billed murres (U. lomvia), were studied in the laboratory. Post-absorptive resting metabolic rates were similar in both species, averaging 7.8 W.kg-1, and were not different in air or water (15-20 degrees C). These values were 1.5-2 times higher than values predicted from published allometric equations. Feeding led to increases of 36 and 49%, diving caused increases of 82 and 140%, and preening led to increases of 107 and 196% above measured resting metabolic rates in common and thick-billed murres, respectively. Metabolic rates of both species increased linearly with decreasing water temperature; lower critical temperature was 15 degrees C in common murres and 16 degrees C in thick-billed murres. Conductance (assuming a constant body temperature) did not change with decreasing temperature, and was calculated at 3.59 W.m-2 x degrees C-1 and 4.68 W.m-2 x degrees C-1 in common and thick-billed murres, respectively. Murres spend a considerable amount of time in cold water which poses a significant thermal challenge to these relatively small seabirds. If thermal conductance does not change with decreasing water temperature, murres most likely rely upon increasing metabolism to maintain body temperature. The birds probably employ activities such as preening, diving, or food-induced thermogenesis to meet this challenge.