TESTIS-DEPENDENT AND TESTIS-INDEPENDENT EFFECTS OF PHOTOPERIOD ON VOLUMES OF SONG CONTROL NUCLEI IN AMERICAN TREE SPARROWS (SPIZELLA ARBOREA)

Songbirds exhibit seasonal changes in the volumes of song control nucl ei. Birds on long, spring-like days have larger nuclei than do birds o n short, winter-like days. The mechanisms mediating volumetric changes have not been determined unequivocally, but testosterone (T) is proba bly involved. This study examined whether testicular factors are uniqu ely responsible for seasonal changes in the song system, or whether ph otoperiod has testis-independent effects. Male American tree sparrows were exposed to one of three photoperiodic conditions: (1) Photosensit ive birds were retained on short days (8L:16D). Plasma T is rarely det ected in such birds. (2) Photosensitive birds were moved from short da ys to long days (20L:4D) and photostimulated for three weeks. Photosti mulation elevates circulating T in photosensitive birds. (3) Photorefr actory birds were held at least four months on 20L:4D. Such birds seld om have detectable levels of T, even though they are on long days. In each condition, there were both intact and castrated birds. Castration typically removes circulating T in tree sparrows, The volumes of the high vocal center (HVC), nucleus robustus archistriatalis (RA), and ar ea X were measured. Song nuclei were largest in intact photostimulated birds. Other long-day birds (i.e. castrated photostimulated, and inta ct and castrated photorefractory groups) had larger song nuclei than d id short-day intact or castrated photosensitive birds and did not diff er from each other. These data indicate that photoperiod has both test is-dependent and -independent effects on the volumes of song control n uclei.