Quick Questions, Short Answers - No Debate

I've long wondered why some birds run/walk and others hop. It doesn't seem to correlate with their feeding habits or habitat in an obvious way. For example, some ground feeders like robins and blackbirds hop, while wagtails and pigeons tend to run or walk. Even starlings walk on the ground, even though they spend a lot of time in trees where they would have to hop. Crows do both more regularly then other birds I have seen.Any ideas?