Moulting is the manner in which an animal routinely casts off  an outer layer or covering, either at specific times of the year, or at specific points in its life cycle. Moulting can involve shedding the epidermis (skin), pelage (hair, feathers, fur, wool), exoskeleton or other external layers. Moulting occurs when animals grow in size (arthropods such as insects) or seasons arrive (birds).