Unlike poikilotherms, birds' body heat allows them to maintain constant body temperature, thus they are HOMEOTHERMS. Poikilotherm body makes heat too, but metabolic rate low and what heat is produced is quickly lost, thus it depends on environment for body heat.
HOW DO HOMEOTHERMS MAINTAIN CONSTANT BODY TEMPERATURES?
1) control heat loss by insulation (fur, feathers).
2) high energy metabolism--they can expend internal energy to increase heat production. You can get a broad, U-shaped graph of oxygen consumption in warm-blooded animals. On either side, lots of energy is required to maintain body head. Across the broad middle of the graph, you have a THERMAL NEUTRAL ZONE (17-40 Degrees C). This is a range of temperature in which energy is not expended to maintain body temperature. The bird is able to use behavior or insulation. Beyond this zone, energy is expended.
At the LCT--the lower critical temperature (where curve starts going up)-- a bird needs to produce heat, it: metabolizes fat, shivers, eats more food and requires additional energy sources. At the UCT--upper critical temperatutre (where curve starts going up)-- an animal must produce water for evaporative cooling or pant to increase air of respiratory surfaces.
EVOLUTIONARILY birds: 1) in cold, lower LCT; 2) in heat, rise UCT and 3) change slope of O2 consumption curve to expend less energy, they DAMPEN UPWARD CURVES.