The antonyms for "one for birds" could be anything contrary to birds, such as mammals or reptiles. Birds are warm-blooded animals that have feathers, wings, and beaks, while mammals are warm-blooded animals that have hair/fur and feed their young with milk. Reptiles, on the other hand, are cold-blooded animals that lay eggs and have scaly skin. Therefore, the antonyms for "one for birds" could be used to refer to any animal that is not a bird, including mammals, reptiles, amphibians, fish, and so forth. When we talk about something that is specifically meant for birds, it becomes important to distinguish it from other animals.