Selective attrition and individual song repertoire development in song sparrows

We compared the songs of young male song sparrows, Melospiza melodia, in the early ‘plastic’ phase (while the songs may still be modified, or dropped altogether) and in the later ‘crystallized’ phase. Birds in this sample dropped 1–3 songs from their plastic repertoire on their way to a crystallized repertoire of 8–11 songs. Consistent with theory of selective attrition, the songs the young birds tended to drop were songs that matched fewer neighbours and matched them more poorly than the songs they retained. At the same time, however, the birds often modified the songs they retained so that these crystallized versions were less similar to the songs of their neighbours than the plastic versions had been. They were more likely to do this for songs they shared with more neighbours. Thus, the song learning process appears to consist of two opposing processes: a tendency to copy and retain the songs of the birds who will be the young bird's neighbours in the first breeding season; and a tendency to modify at least some of these songs so that they are more individually distinctive.