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The male zebra finch sings, whereas the female does not. This behavioral dimorphism is correlated with the presence of morphological sex differences within the neural substrate that mediates this behavior, the song system. When a female chick is exposed to 17beta-estradiol her song system is subsequently masculinized. Either testosterone or 5alpha-dihydrotestosterone may then induce such a female to sing when an adult.
Gurney et al. (Fri,) studied this question.