Birds are a group of warm-blooded vertebrates constituting the class Aves. These birds belong to the category of which of the following?

All warm-blooded animals are homeotherms, irrespective of the environmental temperature they can keep constant body temperature.
Hekistotherms are the plants growing in very low temperature.
Poikilotherms are animals which are cold-blooded and their body temperature fluctuates with respect to the habitat where they live.