Birds were normally raised with their parents until about 40 days post hatch (dph)
after which they were housed individually with no further exposure to tutor song. Hence, we ma e use of the fact that song learning is based on a memory of the father's song which had already been acquired before 40 dph. We investigate the role of NCM in recalling this memory to guide song learning during the sensorimotor period of birdsong development. If NCM stores the tutor song memory used as a template for learning, then the developed songs of NCM-lesioned birds ould be impaired compared to the developed songs in control animals.
At 42 dph the juvenile birds received large bilateral lesions in NCM. Before the lesions, juveniles produced highly variable vocalizations (mean self-similarity score 38.0±6.7, range 26.1N = 5 birds) with little resemblance to the tutor song (mean similarity score 33.0±6.4, range 24.0 39.8, N = 5 birds). At the end of the experiments, birds (76-78 dph) produced highly stereotyped song motifs (mean self-similarity score 63.7±10.0, range 46.7 72.4, N = 5 birds) comparable to song stereotypy in control birds that had not received any treatment (68.0±3.5, range 63.6 72.8, N = 5, p = 0.34, t-test, Figure 1c). Most importantly, motifs of lesioned birds were good copies of their tutors' motifs: The mean similarity score in NCM-lesioned birds at 75 dph was 43.0±8.8 (range 31.1 53.2, N = 5 birds) comparable to the mean similarity score in age-matched controls (48.1±11.0, range 36.2 65.6, N = 5 birds, p = 0.44, t-test, example in Figure 1b, summary in Figure 1c). None of the individual similarity measures (% similarity, sequential match, and accuracy) significantly discriminated by itself between lesioned and non-lesioned birds (p = 0.17, 0.64 0.86, t-test, Figure 1d).
We tested whether the observed increase in motif similarity between NCM-lesioned birds and their tutors could be explained simply by pupils' songs becoming more similar to wild-type zebra finch song rather than specifically to the tutors' songs. We calculated similarity scores between pupils' motifs and those of 14 unfamiliar adult birds, both at the beginning and at the end of the experiment. At 43 dph, the average similarity scores with unfamiliar songs was 27.81±3.86 (range 22.79N = 5 birds), and at the end of the experiments (76-78 dph) the average score was similarly low, 27.62±1.40 (range 26.15 29.02, N = 5 birds, p = 0.92, t-test, Figure 1e), significantly lower than the average similarity score with tutor song (p = 0.002, t-test). In comparison, the similarity between the controls’ songs and the songs of unfamiliar adults was also significantly lower (29.2±3.0, range 26.1 34.2, N = 5 birds) than the similarity between the controls’ songs and tutor songs (Figure 1c, p = 0.006, t-test). This persistence of low similarity scores with unfamiliar songs demonstrates that songs in NCM-lesioned birds developed specifically toward their tutors' songs rather than nonspecifically toward generic zebra finch songs. In summary, despite the very large lesions made, birds successfully developed a good copy of their tutors' songs.