Brain is the major site of estrogen synthesis in a male songbird.

The neural system controlling song in passerine birds can undergo striking morphological and functional changes during both development and adulthood, and many of these changes are regulated by estrogenic hormones. Estrogens circulate at high levels in blood of male songbirds and persist after castration. We measured the activity of aromatase, the enzyme that converts androgens to estrogens, in various tissues from adult male and female zebra finches. As expected, aromatase activity was present in male hypothalamus/preoptic area and pituitary and female ovary, but aromatase was unusually active in whole telencephalon of males and females. By contrast, activity was undetected in testes, adrenals, or other tissues of males. These results suggest that brain is the source of circulating estrogens in the male zebra finch and that estrogen actions on the song system result from local rather than peripheral aromatization.