Naturalists have observed that in a flock of birds, there is usually a strict hierarchy and high-status birds bully low-status birds  The darker the color of the feathers on the head, the thicker the stripes of the chest feathers, the higher the rank, and vice versa  Naturalists also observed that the older the bird is, the darker the feathers on its head are and the thicker the stripes of its chest feathers are
People cannot identify the status of white swans in a group by the color of their head feathers or the stripes of their chest feathers, because the color of their feathers is indistinguishable and their chest feathers have no stripes.
 This shows that the status of birds in a group is gradually established through long-term common life