The big hawk on the fence post is not the one that sat there all summer. The summer hawks have gone south, and rough-legged hawks have come down from the Arctic tundra where they nest to spend the winter on open farmland. For a bird raised north of the treeline, a frozen stubble field is the warm south. Its legs are feathered right down to the toes, which is where both of its names come from (lagopus is Greek for "hare-footed"). The easy way to know one is to watch it hunt: a rough-legged hawk will stop in mid-air and hang over one spot with its wings beating, and it is the only hawk of its size that does that regularly. Their numbers in the north rise and fall with the lemmings and voles they eat, so some winters bring more of them than others.