Vocal control region sizes of an adult female songbird change seasonally in the absence of detectable circulating testosterone concentrations

Precious research established that in several species of seasonally breedin g oscine birds, brain areas [vocal control regions (VCRs)] that central voc al behavior learning and expression exhibit seasonal plasticity, being larg er during than outside the reproductive period. In adult males, this season al decrease correlates with circulating testosterone (T) concentrations. VC Rs contain androgen receptors and T plays an important role in neural plast icity and in the control of singing behavior. In behaviorally dimorphic spe cies, VCRs are larger in males than females and change seasonally also in f emales, but the dependency of these changes on circulating T levels in fema les has not been established. In free-living adult dark-eyed juncos (Junco hyemalis), a species in which females do not normally sing, the sizes of th ree VCRs (high vocal center, robust nucleus of the archistriatum, and Area X) were larger in males than females and decreased between summer and fall in both sexes. In males, this decrease was associated with changes in circu lating T concentrations. Females, however, had on average undetectable T le vels throughout the breeding season. Seasonal changes in VCR volumes in adu lt females may depend on very low (below detection limit) circulating T con centrations, on nonandrogenic plasma steroids, on androgen (or androgen met abolites) produced in brain tissues, and/or on nonsteroidal factors such as photoperiod or social interactions with conspecific birds. (C) 2000 John W iley & Sons, Inc.