Fig. 7. (A) Temperature differences influencing the heat gained from the skin by the external surface of the flight plumage on a pigeon's wing (T_SIW-T_PFW) and the loss of this heat to the surroundings by convection (T_PFW-T_AIR) and radiation (T_PFW-T_ASK) at T_AIR=T_ASK=25°C. (B,C) Predicted effects of lowering both air (T_AIR) and artificial sky (T_ASK) temperatures (B) or only T_ASK (C). The wing temperatures are those of the external surface of the plumage (T_PFW) and of the skin (T_SIW). At low values of T_ASK (T_AIR-T_ASK>7°C), the temperature difference across the boundary layer of air (T_PFW-T_AIR) is reversed and now favors heat gain by the plumage surface through convection.