A bivouac shelter is any of a variety of improvised camp sites such as those used in scouting and mountain climbing. It may often refer to sleeping in the open with a bivouac sack, but it may also refer to a shelter constructed of natural materials like a structure of branches to form a frame, which is then covered with leaves, ferns, and similar material for waterproofing and duff (leaf litter) for insulation. It is sometimes called a bivvy for short.