What kind of bird is a penguin, despite its inability to fly?

A penguin is a flightless bird.

Penguins retain the defining characteristics of birds, including feathers, beaks and egg-laying reproduction, but their wings evolved into stiff, flattened flippers. These flippers provide efficient underwater propulsion rather than the lift needed for aerial flight.

Penguins are not the only flightless birds; ostriches, emus, cassowaries and kiwis are other examples. Penguins differ from most flightless land birds because they are highly specialized marine predators. Their streamlined bodies, dense bones and waterproof feathers help them dive and swim through cold water.