Columbiformes (doves and pigeons) are remarkable among the bird world in that the parents produce a substance called "crop milk". The crop wall undergoes changes while the parents are incubating their eggs so that upon hatching, the squabs can be fed a high protein, high fat material composed mainly of material shed from the parents' crop walls. As the squab mature, their need for this material decreases and eventually disappears, leaving the squab with nutritional needs like those of parrots. During this time the parents' crop walls return to normal.