Fig. 1: Model description.

a The variation of fitness with beak size. In the hypothetical environment under consideration, there are two types of food resources, A and B. An individual with a small beak best utilizes resource A; while resource B is most effectively utilized with a larger beak. b, c Behavior of the males and females. In the bird population under consideration, the males invest a portion of their fitness to make an ornament, to attract the females. Increasing investment in making this ornament comes with costs, including exposure to predators and a physiological disadvantage. Females, on the other hand, are choosy. They first court the males in the population and then pick a mate based on his ornament intensity. A highly choosy female may not find a partner.