The sensitive period for auditory-vocal learning in the zebra finch: Consequences of limited-model availability and multiple-tutor paradigms on song imitation.

Male zebra finches, Taeniopygia guttata, acquire their song during a sensitive period for auditory-vocal learning by imitating conspecific birds. Laboratory studies have shown that the sensitive period for song acquisition covers a developmental phase lasting from 25 to 65days post hatch (dph); formation of auditory memory primarily occurs between 25 and 35dph. The duration of the sensitive period is, however, dependent upon model availability. If a tutor is not available early in development, birds will learn from an adult male introduced to their cage even after they reach 65dph. Birds who are exposed to a second tutor as late as 63dph can successfully adjust their song 'template' to learn a new song model. However, if second-tutor song exposure occurs after 65dph, learning of a new tutor's song will not occur for most individuals. Here, we review the literature as well as novel studies from our own laboratory concerning sensitive periods for auditory memory formation in zebra finches; these behavioral studies indicate that there are developmental constraints on imitative learning in zebra finches.

Mirrored patterns of lateralized neuronal activation reflect old and new memories in the avian auditory cortex.

In monolingual humans, language-related brain activation shows a distinct lateralized pattern, in which the left hemisphere is often dominant. Studies are not as conclusive regarding the localization of the underlying neural substrate for language in sequential language learners. Lateralization of the neural substrate for first and second language depends on a number of factors including proficiency and early experience with each language. Similar to humans learning speech, songbirds learn their vocalizations from a conspecific tutor early in development. Here, we show mirrored patterns of lateralization in the avian analog of the mammalian auditory cortex (the caudomedial nidopallium [NCM]) in sequentially tutored zebra finches (Taeniopygia guttata​) in response to their first tutor song, learned early in development, and their second tutor song, learned later in development. The greater the retention of song from their first tutor, the more right-dominant the birds were when exposed to that song; the more birds learned from their second tutor, the more left-dominant they were when exposed to that song. Thus, the avian auditory cortex may preserve lateralized neuronal traces of old and new tutor song memories, which are dependent on proficiency of song learning. There is striking resemblance in humans: early-formed language representations are maintained in the brain even if exposure to that language is discontinued. The switching of hemispheric dominance related to the acquisition of early auditory memories and subsequent encoding of more recent memories may be an evolutionary adaptation in vocal learners necessary for the behavioral flexibility to acquire novel vocalizations, such as a second language.