The crest, in some cases called crown, is a group of feathers that some species of birds have on top of the head. The feathers of these crests can move up and down or be permanently up, depending of the species. The crest is made up of semiplume feathers: a long rachis with barbs on either side. These are plumulaceous feathers, meaning that they are soft and bendable. In birds, these semiplumes are common along the head, neck, and upper back, and may be used for buoyancy and sensing vibrations. Many domesticated bird species have crest feathers. These structures are known to have two origins: selective breeding or mutations. The English Budgie is an example of a crest obtained by selective breeding.