Fig. 3.

A, Castration did not alter dramatically the amount of singing in the fall. The mean number of songs per hour was similar in intacts and castrates. B, Castrates (○) and intacts (■) differed markedly in blood T levels, but the number of songs produced per hour by birds in both groups showed broad overlap. Blood T levels, which were below detection in all castrates, are shown here at an arbitrary point between 0 and 0.5 ng/ml. C, Intacts and castrates had similar levels of circulating T before gonadectomy was performed on the latter birds, but these levels differed markedly after castration.