Penguins have more feathers than most other birds to keep them warm in the seawater. BUT, feathers wear out and despite preening they must be replaced.
The feathers have two layers, which are waterproof, short, stiff and hooked enabling them to lock together, trapping a layer of air between the skin and the feathers providing a waterproof insulating layer.
Moulting is essential because feathers wear out during the year. The feathers become worn when penguins rub against each other, come in contact with the ground and water, and regularly preen (clean, rearrange, and oil) their feathers.
The old feathers of moulting penguins do not fall out until the new ones have partially grown underneath, so without their full strength of their insulation the penguins must remain on land.
Whilst moulting, the birds virtually have holes in their diving suits and would drown if they went to sea.
During this period of moult when they are forced to stay on land, they cannot eat and are dependent on their stored fat. They look really scruffy and lose up to half their body weight – a whopping 3-4kg (of 5 to 8 kg).