THE SEARCH FOR NEURAL MECHANISMS THAT DEFINE THE SENSITIVE PERIOD FOR SONG LEARNING IN BIRDS

Vocal learning is the modification of vocal output by reference to auditory information. We do not know how auditory information is acquired and we do not know how it modifies vocal output. But we know that there are special times during development and in adulthood when vocal learning occurs, and other times when it does not occur. This is a review of what we know about those times. The review highlights behavioural, hormonal, anatomical, physiological and molecular information. I pay particular attention to what is known about the brain pathways used by songbirds for the acquisition and production of learned song, and their relation to auditory pathways. Factors that enable or block vocal learning must act on those auditory and motor pathways, or on pathways that control the motivation for vocal learning. Stages in the development of the song system probably determine the timing of stages in vocal development and learning-and thus influence the first occurrence of food begging calls, subsong, plastic song and stable song. Vocal learning occurs primarily at times when the pathways for vocal learning are growing and forming connections and plasma testosterone levels are low. Parts of this process can recur in adulthood, possibly as a result of neurogenesis and neuronal replacement in some key song nuclei. All the material reviewed here comes from oscine songbirds, mostly zebra finches and canaries.