Figure 1.

(A) Schematic illustrating the major projections within the song production circuit. Note the ventricular zone (VZ) in black directly dorsal to HVC. RA = robust nucleus of the arcopallium, nXII = tracheosyringeal portion of hypoglossal nucleus, nAM/nRAm = nucleus ambiguus and nucleus retroambigualis, D = dorsal, and R = rostral. HVC grows and regresses each breeding and nonbreeding season by about 68,000 neurons as a result of increased new neuronal survival and neuronal apoptosis, respectively. (B) Schematic illustrating how the balance between neuronal birth and death hypothetically contributes to HVC growth and regression. (C) Predictive models for HVC neuronal turnover. New neurons that enter HVC might: entirely replace oder neurons (left), temporarily contribute to HVC growth and then die (middle), or serve to replace some older neurons, but also die (right). (D) Experimental design for testing the predictive models above in nonbreeding (i.e., top) and breeding (bottom) conditions. SD = short day photoperiods to promote nonbreeding physiological condition (light grey), LDT = long day photoperiods and exogenous testosterone pellet to stimulate breeding-like physiological condition (dark grey), TA1 and TA2 = thymidine analog 1 and thymidine analog 2. Note: darker grey TA2 background (bottom) indicates injections were administered in LDT conditions rather than SD conditions.