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
If there is a strict hierarchy in the bird world, then in a flock of birds, they will also struggle to improve their status.
 This shows that the status of birds in a group is gradually established through long-term common life