Mimicry
Biologists still do research on Batesian and Müllerian mimicry. They study how the models differ in their foul taste; and what happens when the ratio of mimics to models varies. Quite often, it is only the female which is a mimic; the male carries the normal appearance of its genus. The females need more protection, while the males need to mate. A more subtle reason is that it halves the number of mimics, and so bolsters the effectiveness of the mimicry. Batesian mimicry might damage the warning effect if the frequency of mimics became high, because more young birds would taste them and be encouraged to try again. The benefit of warning declines if there are more mimics.