The melting snow has softened the ground into mud, luring out of the earth a host of worms. This is the robins’ favorite season. These ground-dwelling thrushes mainly survive on earthworms, as they aren’t designed to digest seeds. They come back North at the same time as the earthworm migration. (It sounds crazy, but worms do migrate. However, they migrate vertically: in the winter they dig deep under the ground & huddle up until spring.) The robins sense the worms reappearing, & it brings them in flocks. Some robins do stay the winter, feeding on whatever fruit or berries they can find. In the spring then, it’s obvious which robins stayed & which flew south, for the migrating robins are always significantly plumper than their wintering cousins.