Abstract
The song control nuclei of the brain of the male zebra finch (Poephila guttata) contain more neurons of larger cross-sectional area and more extensive innervation than those in the female. The sexual dimorphism of nucleus robustus archistriatalis (RA), one of the song nuclei, arises from neuronal atrophy and death in the female brain as well as from an increase in cell-body size and afferent terminals from other forebrain nuclei in the male. Subcutaneous implantation of a slow-release pellet of oestrogen in a female chick not only prevents her RA neurons from dying but also promotes their growth and innervation. The masculinizing effects of oestrogen on RA appear to be limited to a period of 35-40 days after hatching. Oestrogen may act on RA neurons indirectly by promoting their innervation from other forebrain song nuclei.
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