A fundamental feature of nervous systems is that they provide plasticity of structure and function which allows animals to adapt to changes in their environment. We are only beginning to understand the underlying mechanisms, the limitations, and the behavioral consequences, such as learning, of naturally occurring plasticity in the brain. The song control system in the avian brain provides excellent opportunities for addressing such basic issues of behavioral neuroscience. Seasonal changes of environmental factors, such as photoperiod, have a profound effect on birds and most other animals. In songbirds seasonal changes in photoperiod elicit changes in circulating concentrations of gonadal steroid hormones, which in turn cause changes in song behavior and in morphological and physiological attributes of the neuroendocrine system that controls song. The goal of this proposal is to use a comparative approach to explore the proximate mechanisms and behavioral functions of seasonal plasticity in the avian song control system. The time course of changes in the morphology of song nuclei following changes in seasonal photoperiod will be examined in male canaries. Autoradiographic techniques will be used to determine whether seasonal changes in the cellular accumulation of gonadal steroid hormones underlie changes in the morphology of song nuclei in canaries. To test the hypothesis that seasonal changes in song nuclei depend on innervation from afferent nuclei, we will lesion nucleus HVC unilaterally in canaries and compare the morphology of the efferent nuclei RA and Area X ipsilateral and contralateral to the lesion following exposure to a breeding photoperiod. Song behavior, hormone concentrations, and song nuclei will be measured in wild Nuttal's white-crowned sparrows during and after the breeding season to test for seasonal changes in this subspecies. Using wild male song sparrows, song behavior, hormone concentrations, and song nuclei will be measured at four different times of year to test the hypothesis that seasonal changes in song production correlate with seasonal plasticity of the song nuclei. The results of the proposed studies will increase our understanding of steroid hormonal influences on the nervous system, and of the relationship between plasticity in the adult brain and learning.