Energetics and Spread-Winged Behavior of Anhingas in Florida

Metabolic rates and body temperatures of four captive Anhingas (Anhinga anhinga) were measured over a range of ambient temperatures. In addition, Anhingas were observed in the field in an attempt to correlate the frequency of behaviors, such as wing-spreading, with environmental conditions, such as the intensity of solar radiation and ambient temperature. Anhingas have low basal metabolic rates and high thermal conductances for their mass. The frequency of spread-winged behavior increases at higher solar intensities and is inversely correlated with ambient temperature. Anhingas assuming spread-winged postures orient with their backs to the sun, maximizing the surface area exposed to insolation and maintaining an angle of incidence of approximately 90°. In contrast, birds engaged in gular flutter face into the sun, thereby minimizing the surface area exposed to insolation and increasing the angle of incidence. Anhingas spread their wings to dry the plumage and to absorb solar energy during cool weather. They thereby supplement their low metabolic rates, compensate for their high thermal conductances, and replace heat lost via evaporation and convection due to wetting of the plumage.