Seasonal plasticity of structure and function is a fundamental feature of nervous systems in a wide variety of animals that occupy seasonal environments. Excellent examples of seasonal brain changes are found in the avian song control system [HVC], which has become a leading model of morphological and functional plasticity in the adult CNS.
The volumes of entire brain regions that control song increase dramatically in anticipation of the breeding season. These volumetric changes are induced primarily by vernal increases in circulating sex steroids and are accompanied by increases in neuronal size, number and spacing.
In several species, these structural changes in the song control circuitry are associated with seasonal changes in song production and learning. Songbirds provide important insights into the mechanisms and behavioral consequences of plasticity in the adult brain.
…In the most extreme example, the volume of HVC in spotted towhees (Pipilo maculatus) nearly triples between the non-breeding and breeding seasons (Figure 2). This naturally occurring plasticity in the songbird brain is perhaps the most pronounced observed in any adult vertebrate. One should note, however, that not all of the song nuclei exhibit seasonal volumetric changes
