The Winter Moth caterpillar is quite small, this one was about a centimetre in length. they can move around between trees by hanging from a a silk thread and getting blown in the wind. They feed on leaves (and so can be unwelcome) but they make good food for birds! This one was found in bird nesting season, so would be ideal food for chicks. In fact some birds time their breeding to coincide with the emergence of these caterpillars.