Down feathers are the soft, fluffy feathers that first appear on young birds and that also form the protective undercoat of many avian adults. Slightly different in structure, down feathers do not feature the interlocking barbs that help give other types of feathers their strength. However, since their primary function is to provide warmth and insulation, their relative weakness is inconsequential. In fact, down feathers are so well-suited to their role as insulators, humans often utilize them to stuff winter coats and bedding, such as comforters and sleeping bags.