Alder Moth Caterpillar (Acronicta alni)
What a great find today!
This moth (caterpillar) is thinly scattered over most of England and Wales - less so in Scotland. The larva when young resembles a bird dropping, but when mature is highly distinctive, being black and yellow with a number of club-like hairs. It feeds on a number of different trees, particularly alder (Alnus glutinosa) and birch (Betula).