Naturalists have observed that there is usually a strict hierarchy in a flock of birds, with the higher birds bullying the lower ones  The darker the head feather and the thicker the breast feather stripes, the higher the rank, and vice versa  Naturalists have also observed that the older the bird, the darker the head feathers and the thicker the stripes of the breast feathers
If there is a strict hierarchy in the bird world, then within a group of birds, they will also fight to improve their status
 This suggests that the status of birds in a group is gradually established by living together for a long time