Cytoarchitecture of vocal control nuclei in nestling budgerigars: Relationships to call development

Changes in the cytoarchitecture of vocal control nuclei were investigated i n nestling budgerigars (Melopsittacus undulatus) from hatching to fledging (five to six weeks) in relation to changes in vocalizations produced by nes tlings during this period. The nuclei investigated were the hypoglossal nuc leus, dorsomedial nucleus of the intercollicular midbrain, central nucleus of the archistriatum, central nucleus of the lateral neostriatum, oval nucl eus of the hyperstriatum ventrale, medial division of the oval nucleus of t he anterior neostriatum, and magnocellular nucleus of the lobus parolfactor ius. These nuclei have been shown to form functional circuits in adults rel ated to vocal learning. Consistent with previously reported results, we fou nd that call development could be described in terms of five different phas es based on changes in the duration and segmentation of single and multiple segment foodbegging calls and the appearance of the first socially learned contact calls around the time of fledging, We also found that call segment duration exhibited an inverted U-shaped developmental function during the nestling period, as has been found for total call duration. Cytoarchitecton ic studies revealed striking changes in the cellular architecture of vocal control nuclei during the first four weeks posthatching. At hatching the hy poglossal nucleus exhibits adult-like cytoarchitecture, and the central nuc leus of the archistriatum and the central nucleus of the lateral neostriatu m are distinguishable from surrounding fields. By one week posthatch, the c entral nucleus of the archistriatum exhibits an adult-like appearance, whil e other telencephalic vocal control nuclei do not exhibit adult-like cytoar chitecture until three to four weeks posthatching. By two weeks posthatchin g, the dorsomedial nucleus of the intercollicular midbrain also exhibits ad ult-like cytoarchitecture. We observed substantial decreases in the thickne ss of ventricular proliferation zones during this period, with decreases in ventricular zones occurring at about the same point that nuclei at corresp onding levels come to exhibit adult-like cytoarchitectonic features. Of int erest is the fact that cytoarchitectural development occurs asynchronously in different brain regions, with the appearance of adult-like characteristi cs in the hindbrain and midbrain occurring before the appearance of adult-l ike cytoarchitectonic characteristics in telencephalic nuclei. These result s are consistent with recent lesion studies indicating that neither auditor y feedback nor telencephalic vocal control nuclei are necessary for the pro duction of foodbegging and other nestling calls until three to four weeks p osthatching.