Core and skin temperature were measured by radiotelemetry in starlings (Sturnus vulgaris) during 30 min flights in a wind tunnel. Core temperature was independent of ambient temperature from 0 to 28 degrees C. The temporal mean of the monitored core temperature during flight was 42-7 degrees C in one bird and 44-0 degrees C in another. These temperatures are 2-4 degrees C higher than the resting temperature in starlings, and are among the highest steady-state temperatures observed in any animal. Skin temperature on the breast was within a few degrees of core temperature. In some locations skin temperature was higher at low ambient temperatures than at intermediate ambient temperatures. An analysis of the data shows that a high core temperature does not function as an aid to head dissipation. On the contrary, insulation is adjusted to maintain a high temperature, presumably because it is necessary for flight. The increase in skin temperature at low ambient temperatures is believed to be a result of a decrease in heat flow through the breast feathers brought about by feather adjustments, to compensate for an unavoidable increase in heat flow in unfeathered or poorly feathered parts of the body.