Thermoregulatory Behavior during Incubation in Heermann's Gulls

Simultaneous behavioral observations and microclimatic observations indicate a tight coupling between the daytime incubation behavior of Heermann's gulls (Larus heermanni) and thermal conditions in the rookery. By means of thermoregulatory behavior, these gulls are able to maintain their own thermal balance while protecting their eggs from overheating on the Mexican desert islands where they breed. The various components of their behavioral thermoregulation facilitate either heat retention or heat loss through modification of convective heat loss and evaporative cooling. These behavioral components were assigned arbitrary numerical values, and a thermoregulatory index ranging from -1.0 (heat retention) to 8.0 (maximum heat loss) was computed for a variety of thermal conditions. This index was most strongly correlated with substrate temperature (RHO = .87), evidently because this physical parameter integrates the thermal conditions in the rookery in a manner similar to that of the gulls themselves.