Anatomical and ontogenetic factors producing variation in HVc neuron number in zebra finches

The volume of nucleus HVc of the avian song system varies greatly both amon g and within songbird species, and is positively correlated to song complex ity in many species. Moreover, the number of neurons in HVc predicts the ab ility of individual zebra finches to imitate song accurately. To better und erstand this brain/behavior relationship, we used the retrograde tracer Fas t Blue to assess how specific HVc neuronal subpopulations contribute to var iation in overall HVc neuron number in adult male zebra finches. We also in vestigated whether sibling order predicts the number of HVc neurons and/or yolk levels of testosterone, a hormone that might regulate the development of HVc. We report that total HVc neuron number is consistently and independ ently predicted by the size of each of its two projection populations, sugg esting that the proportional makeup of HVc is rightly regulated at least in male zebra finches. Also, while we failed to detect a significant effect o f sibling order on either the number of neurons in HVc or yolk testosterone concentration, we found that clutch of origin made a large contribution to variation in both early hormone levers and HVc neuron number. (C) 2001 Els evier Science B.V. All rights reserved.