Instead, I was pleasantly surprised to discover a mourning cloak butterfly (Nymphalis antiopa).
It's so well camouflaged that the edges of its wings are barely noticeable against the wood.
While most butterflies migrate or overwinter as eggs, larva or pupae, adult mourning cloaks find a spot under tree bark, in a hollow or another suitable crevice such as eaves of a house, where they remain through the winter in a state of diapause. Like other creatures that overwinter, mourning cloaks are able to keep from freezing by building up glycerol, an "anti-freeze" in their bodies while also lowering their metabolic rate. It's amazing that such a fragile looking creature can withstand the fury of winter. By overwintering, mourning cloaks gain by being the first butterflies of spring. They may be seen drinking tree sap. The females will lay eggs on host trees - birches, poplars and elms.