Bird health is vital to ensure they are able fly, to compete with others for food, reproduce and a variety of other purposes. Feathers are of ultimate importance to birds, and though they can be replaced, if they are to function properly they have to be kept in good condition. Individual feathers are frail things and birds spend a considerable amount of time caring for them. In many water birds the plumage (i.e. all the feathers) is waterproof. In some species this waterproofing is assisted by oil supplied by special glands 'uropygial glands'. The birds apply the oil to their feathers during preening. An action you often see birds performing and which involves the bird using its bill to manipulate its feathers to ensure they are all zipped up and lying flat, one on the other like tiles on a roof of a house. There are two main activities involved in feather care:- Preening and Bathing and several minor ones such as sunning, anting.