The correct characteristic of birds among the options given is "They have fast metabolisms." Birds are warm-blooded (endothermic) animals, meaning they maintain a constant internal body temperature, and they typically have high metabolic rates to support their energy needs, especially for activities like flying.
The statement "They have two limbs" is misleading in the context of birds, as they actually have two wings and two legs, while "They are cold-blooded" is incorrect since birds are warm-blooded. Therefore, the option "All of these are correct" is also incorrect.