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
The position of a white swan in a colony cannot be identified by the color of its head feathers or the stripes of its chest feathers, which do not distinguish between dark and light colors
 This suggests that the status of birds in a group is gradually established by living together for a long time