While they might look and act differently than other birds, they have all the characteristics of birds:
* Feathers: Penguins have feathers, though they are modified for swimming.
* Wings: Penguins have wings, but they use them for swimming instead of flying.
* Lay eggs: Penguins lay eggs, which is a characteristic of all birds.
* Warm-blooded: Penguins are warm-blooded, another characteristic of birds.
So, even though they swim and live in cold environments, penguins are still birds!