Birds have feathers which form a flight surface and provide insulation for warmth. A flight surface is the surface over which air flows to provide lift and control for the bird. Feathers are made of the protein keratin, as are claws, fingernails, hair, fur, wool, porcupine quills, and bird beaks. Keratin contains the sulfur containing amino acid cystine and the disulfide bond provides some strength and stiffness to keratin. This accounts for the sulfur peak in the PIXE spectrum. Feathers consist of interlocking strands radiating from a central shaft. This shaft is sometimes filled with cells like cork to keep it stiff.