This is a very common fungus found on the Birch (Betula) species. It has a distictive shape and colouring. The upper surface is grey/brown with a leathery texture. The underside is white. The bracket may be connected to the stem with a stalk. These fruiting bodies are a reliable sign that the affected area has an advanced level of infection and is probably extensively decayed. Such infected branches and stems can fail in windy conditions.