Fig. 6. Examples of melanin pigment patterns produced by experimental manipulation (A–D) and computer simulation (E,F). (A,B) Ornamental feather from the neck region of a male Brown Leghorn chicken receiving 1.5 mg of a subcutaneous injection of thyroxin every sixth day during growth of the feather (Lillie, 1932) (A); (B) control feather from the same region. (C,D) Normal feather from the wing covert of a short eared owl (C), and (D) a feather from the same covert induced to grow after removal of the thyroid gland [from Voitkevich (Voitkevich, 1966), p.183]. (E,F) Two patterns produced in computer simulations of diffusion and interactions between two molecules (`activator' and `inhibitor') on a growing feather (Prum and Williamson, 2002). The activator stimulates production of both activator and inhibitor. Inhibitor diffuses faster than, and inhibits, activator. Dark areas are locations where the activator is above a certain concentration. One important difference between the two simulations is that the diffusion rate of inhibitor is higher when the longitudinal patch is produced. Both these patterns are commonly observed across feather tracts of different species of birds (Prum and Williamson, 2002).