Developmental changes in the cellular composition of a brain nucleus involved with song learning in zebra finches.

Using a double-labeling technique to characterize projection neurons and androgen target cells, we examined ontogenetic changes in the cellular composition of IMAN, a forebrain nucleus that plays an important role in song learning during a restricted period of male zebra finch development. This nucleus undergoes a massive loss of neurons during the time of song acquisition. We report that during the period of cell loss in IMAN, neither the property of projecting to an efferent target nor the ability to concentrate androgens is able to spare neurons from ontogenetic cell death. Furthermore, we report that, at the time when IMAN ceases to influence song production, a large proportion of androgen-sensitive cells that do not make an efferent projection lose the ability to accumulate androgens.