DIVING METABOLISM AND THERMOREGULATION IN COMMON AND THICK-BILLED MURRES

The diving and thermoregulatory metabolic rates of two species of divi ng seabird, common (Uria aalge) and thick-billed murres (U. lomvia), w ere studied in the laboratory. Post-absorptive resting metabolic rates were similar in both species, averaging 7.8 W.kg-1, and were not diff erent in air or water (15-20-degrees-C). These values were 1.5-2 times higher than values predicted from published allometric equations. Fee ding 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 re sting metabolic rates in common and thick-billed murres, respectively. Metabolic rates of both species increased linearly with decreasing wa ter 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 temperatu re, and was calculated at 3.59 W.M-2.-degrees-C-1 and 4.68 W-M-2.-degr ees-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 condu ctance does not change with decreasing water temperature, murres most likely rely upon increasing metabolism to maintain body temperature. T he birds probably employ activities such as preening, diving, or food- induced thermogenesis to meet this challenge.