Abstract
The development of birdsong and development of human speech share some features; both are complex sounds used for communication, and learning is acquired and restricted to youth. Song learning takes place in two periods, a sensory phase of song memorization and a sensorimotor phase of producing from the memory. The stage of song development where song becomes stable and stereotyped is called crystallization. Thereafter, adult birds usually do not learn new song.
It is not known which stage impedes adult learning, however. We first postponed the sensorimotor phase using a noise-masking method and analyzed the songs over a long time period. The memory of song phonology persists well after the normal critical period, and the adult birds were able to use it to shape their songs and even retune their drifted songs. However, they never amended the syntax. Thus, the sensorimotor phase for phonology is not restricted to youth, and different schedules and processes may govern the learning of syllable phonology and syntax.
Next, we deprived young birds of the model song and postponed the sensorimotor phase, to examine whether the sensory phase is limited to youth. Adult birds even without previous experience of hearing or singing a song failed to learn a song. Thus, neither song memorization nor song crystallization terminates the sensory period, and age per se limits the ability or motivation to learn song.
