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I would say muscles. All birds can raise their head feathers, but some birds are designed slightly differently than others. Cockatoos have what appears to be extra "skin" on the top of their head where their long crest feathers grow as adults. Then you have hawk heads. Talk about a beautiful prehistoric looking creature!
At the base of each feather follicle there's actually a tiny muscle that attaches to the feathershaft. Super cool when you think that to raise their whole crest, cockatoos are contracting all of these little muscles at once!