Birds are warm blooded, which enables them to maintain a body temperature of roughly 40 C regardless of the surrounding heat levels. The key to maintaining a stable temperature is that heat production should be balanced by heat loss.
When the external temperature is high a bird may elevate the rate of heat loss by increasing the blood supply to its legs and areas of bare skin, which are well served with blood vessels.
When the external temperature is close to or rises above body temperature, it is more difficult to dissipate heat in this way. This is when cooling through the evaporation of water becomes important, something humans achieve with sweat glands.
Birds do not have sweat glands, but can evaporate water through their respiratory tract. Watch a bird on a hot summer's day and you may see it panting or, in some species, rapidly moving the floor of its mouth (termed a 'gular flutter'). Behavioural responses, such as seeking shade, are also important.
Main image: Birds bathing. © Getty