Environmental Influences on the Energetics and Behavior of Anhingas and Double-Crested Cormorants

The energetics of anhingas and double-crested cormorants were measured and their behavior monitored under an array of environmental conditions. Anhingas have lower basal metabolic rates and higher thermal conductances, and doublecrested cormorants have higher basal rates and higher thermal conductances than the expected values as determined from Aschoff and Pohl (metabolic rate) and Aschoff (thermal conductance). Both species have lower basal metabolic rates, body temperatures, and thermal conductances at night, and elevated basal metabolic rates and rates of evaporative water loss under incident radiation, as compared with daytime situations without incident radiation. Both species have elevated metabolic rates and thermal conductances while wet. Double-crested cormorants exhibited spread-winged postures only when wet; anhingas readily displayed them regardless of plumage condition. Thermal conductance in cormorants is unchanged during exposure to incident radiation, but anhingas use spread-winged postures to enhance insolation absorption, thereby reducing their rates of heat loss and metabolic rates at low ambient temperatures and extending the thermoneutral zone. These physiological and behavioral differences between anhingas and double-crested cormorants may be responsible for the difference in their geographic distributions.