Hazel dormice have golden-brown fur and large black eyes and, distinctively, they are the only small British mammal with a furry tail. They are nocturnal and spend almost all of their time in the branches of trees during the summer, rarely coming down to the ground. They have sometimes been found asleep in old bird nests but they weave their own nests (often in brambles or other shrubs) from strips of honeysuckle bark or a similar plant, surrounded by a layer of green leaves. When conditions are cold or wet, or if food is scarce, dormice curl up into a ball and go into a state similar to hibernation for a short time (called torpor) in order to save energy. Between October and May dormice hibernate in nests beneath the leaf litter on the forest floor or in the base of hedgerows.