All birds must get rid of old worn feathers and grow new ones from time to time. This is done all at once (in a few weeks) and is called moulting. All male ducks that come from the Mallard (all domestic ducks except the Muscovy) go through two moults every year. They moult once after the mating season and grow feathers which look like the females. Then they grow new flight feathers and moult once again into their mating colours.
To moult is to shed the outer cover of the body. The stag beetle's larva has a hard shiny head which unlike its body is not elastic at all. So in order to grow it has to shed its outer cover. After each moult it gets bigger and fatter. By the time that it has moulted four times is the head is several times the size it was when it first came out of the egg and the body is the size of a person's thumb.
Moulting (or molting, see spelling differences) is the routine shedding of old feathers in birds, old hairs in mammals (see also coat (dog)), old skin in reptiles, and the entire exoskeleton in arthropods.