Question: Why do some birds hop while other birds walk?
Answer: There is no absolute answer because some birds do both.
Generally, it has to do with a combination of economy of effort and the physiology of the bird.
Short-legged birds tend to hop because they can go farther in a single hop than they can in several steps. It is more energy efficient.
However, it takes less energy for birds with longer legs to stride, like the Great Blue Heron at the left.
Even that may vary depending on the activity. Many short-legged birds walk rather than hop while foraging. Speed also affects the choice; a hopper in a hurry usually breaks into a run.