Polymorphism provides a classic example of adaptive evolution. A great advantage of studying polymorphism is that the phenotype can serve as a genetic marker; therefore, researchers can take full advantage of this to test a hypothesis based on Darwin's principle of adaptive evolution. Evolutionary theories of polymorphism have been established and suggest that polymorphism is likely maintained through negative frequency-dependent selection. Such selection is mediated by the viewer's perception or recognition of the focal trait as expressed by appearance, such as the colour and pattern of eggs. The viewer's response to that trait selects against the majority of the population, and thus favours the rare type. Such evolutionary dynamics can also be applied to avian brood parasitism, in which parasites exploit the parental care of their hosts. In this review, I describe recent findings in brood parasitic systems, in which polymorphism plays an important role in the coevolutionary arms races, at the egg, chick, and adult stages. Finally, I emphasize the need to synthesize traditional ultimate approaches with proximate approaches, incorporating genomics and psychology, in order to draw a fuller picture of the coevolutionary arms race between avian brood parasites and their hosts.