Birds are homeothermic. Their thermoneutral zone lies between 18 and 36°C, but the upper critical temperature is strongly dependent on the relative air humidity (RH%), which is lower at higher RH%, on breed and on production performance. Also the age of the parent stock and brooding conditions affect the heat tolerance of the offspring. As long as the ambient temperature is lower than the birds' body temperature, heat loss from the core to the skin can be increased by radiation, depending on peripheral blood flow. Poultry responds to high environmental temperatures by behavioural changes, which allow them to re-establish heat balance with their surroundings. During periods of heat stress, broilers rest more, stand more quietly or simply sit near walls or waterers. Usually, they lift their wings in order to promote cooling by reducing body insulation. Hyperventilation or "panting" increases during periods of high environmental temperature, leading to increased CO2 loss.