Accumulation of estrogen in a vocal control brain region of a duetting song bird.

Tritiated estradiol (E) was injected into bay wrens (Thryothorus nigricapillus), a tropical species in which females sing complex vocal duets with males. Autoradiographic analysis revealed that males and females have equal proportions of cells labeled by estradiol or its metabolites (E target cells) in a telencephalic region involved in song: the caudal nucleus of the ventral hyperstriatum (HVc). Other forebrain song regions failed to show labeling by E. E or its metabolites were accumulated, however, by cells in the midbrain song region ICo (the intercollicular nucleus) and in hypothalamic regions. This pattern of accumulation in the song system differs from that observed in a previous study in which bay wrens were injected with tritiated testosterone (T); T or its metabolites were accumulated by cells in HVc, RA (robust nucleus of the archistriatum), MAN (magnocellular nucleus of the neostriatum), ICo, and nXII (hypoglossal (nucleus). Such comparison suggests that cells in HVc have different steroid accumulation properties from those in other song regions. Bay wrens differ from zebra finches (Poephilia guttata), in which HVc contains very few E target cells. The wrens are more similar to canaries (Serinus canarius), because both species have E target cells in HVc, and females of both species are able to sing. The interspecies comparison raises the question of whether the ability of HVc cells to accumulate E or its metabolites in both species constitutes a precondition for the bisexual potential for song production.