The correct answer is option 1 - Birds.

Birds are unique among the given options because they are endothermic, meaning they can generate and maintain their own body heat. This ability allows birds to regulate their body temperature independently of the surrounding environment.

Amphibians, on the other hand, are ectothermic, meaning they rely on external sources of heat to regulate their body temperature. They are unable to maintain a constant body temperature and their internal temperature often fluctuates with the environment.

Fishes, like amphibians, are also ectothermic animals. They adapt to the temperature of the surrounding water, which can vary greatly. Their body temperature is largely influenced by their environment.

Reptiles are typically ectothermic as well, although some groups can exhibit limited endothermy. However, they still rely heavily on external sources of heat to regulate their body temperature.

In summary, birds are the only animals from the given options that maintain a constant body temperature in changing environmental conditions due to their ability to generate and regulate their own body heat.