Birds are animals that can maintain a constant body temperature.
The production of heat by the body of birds is related to the ability of food to oxidize, which depends on good tissue oxygenation. This tissue oxygenation is possible thanks to the ventilation of the respiratory system and thanks to the heart, which has four chambers.
Another factor that interferes with the maintenance of body temperature in birds is the presence of subcutaneous fat and a layer of feathers. This layer of feathers helps a lot in controlling the temperature, because when the temperature is low, the feathers shiver, increasing the layer of air trapped between them, promoting thermal insulation. At the same time that this occurs, there is also a greater oxidation of food, to produce more heat, which consequently uses more energy.
In the feet of birds there is a mechanism of counter-blood flow that prevents them from losing heat through these structures. In this, the blood vessels that carry blood towards the feet are surrounded by other vessels that carry blood in the opposite direction, that is, towards the body, and this blood absorbs the heat and brings it back to the body, preventing that there is heat loss.
Due to endothermy, birds and mammals do not need to be exposed to the sun, even when the ambient temperature is low. This is an advantage for these animals that can be more active at night, on cloudy days or in cold climates.