Selective attrition of songs and matched counter-singing

Male sparrows learn more songs ("overproduce") than they need for their final repertoire. In the early stages of practice singing, termed plastic song, hand-reared males of some subspecies may sing up to 8 different imitations of different tutor songs. They then gradually discard all but the one song type they retain for the rest of their lives as their "crystallized" song. This attrition process is selective--it is guided by auditory input during the plastic song stage. If a male hears playback from a loudspeaker of one song type that matches one of the songs in his overproduced repertoire, he will retain that song as his crystallized song, and discard the other song types. Males presented with playback of a novel song did not learn it, but rather crystallized one of the songs they had learned 6-9 months earlier during the sensitive phase. We suggest that instances of song change in yearlings and adults that have been assumed to reflect memorization of novel songs, and hence, an open-ended sensitive phase, may in fact be the result of overproduction of songs memorized early in life, followed by selective attrition due to matched counter-singing.