Why do some pelicans have bumps on there beaks and not others?

The bird's large, bright orange bill makes this species easy to identify. During the breeding season, both males and females

develop a 3 inch by 3 inch (7.6 by 7.6 cm) bump on the top of their large beak. This conspicuous growth, which evidently

indicates the bird's interest in breeding, is shed by the end of the breeding season.