molting, periodical shedding and renewal of the outer skin, exoskeleton, fur, or feathers of an animal. In most animals the process is triggered by secretions of the thyroid and pituitary glands. Nearly all birds molt annually in the late summer, losing and replacing their feathers gradually over a period of several weeks. Except among ducks, rails, and diving birds the ability to fly is not lost. Some birds undergo a second or prenuptial molt in the spring, changing from dull to bright plumage. The development of the young bird is marked by successive molts: first, from the down of the very young to the juvenal plumage, which resembles that of the female in species showing color differences between the sexes; then to the first winter plumage, when the bird is called an immature; and finally to the first nuptial plumage, the adult stage. Arthropods (e.g., insects and crustaceans) must molt their exoskeletons periodically in order to grow; in this process the inner layers of the old cuticle are digested by a molting fluid secreted by the epidermal cells, the animal emerges from the old covering, and the new cuticle hardens. In insects the stages between molts are called instars. Amphibians and snakes usually shed their skins several times a year. Mammals change from heavy winter to light summer pelage. Protective coloration is exhibited in the color changes of such mammals as the ermine and the varying hare and, more dramatically, among such birds as the ptarmigan.