Why do some species of birds hop and others walk?
No one really knows, but some ornithologists think that it has something to do with economy of motion. Smaller birds tend to hop, covering more ground than they would by walking on their little teeny legs. Larger birds tend to walk, covering more ground with less energy than they would by hopping.
That doesn’t explain why different species in the same families differ in their preference, though (for example, jays hop but crows walk. Jays and crows are both members of family Corvidae.)