We made bilateral ibotenic acid lesions in NCM of song-isolate birds when they were 42.5 days old (range 39 to 45, N = 12 birds) and thereafter we exposed the juveniles daily to a singing adult tutor (see Methods). At the end of the experiments (mean 76.3 dph, range 72 to 82 dph, N = 12 birds), juveniles with NCM lesions showed a high song stereotypy (mean 66.9 ± 11.9, range 36.1 to 77.3, N = 12), comparable to the stereotypy in age-matched untreated control juveniles (60.9 ± 7.6, range 53.9 to 70.9, N = 5, p = 0.32, t-test, Panel B).
NCM-lesioned birds successfully imitated tutor song motifs with an average similarity score of 50.7 ± 10.9 (range 29.1 to 65.3, N = 12 birds), which was not significantly different from similarity in control birds and their tutors (average score 45.1 ± 13.0, range 26.8 to 58.3, N = 5 birds, p = 0.37, t-test). None of the individual measures were significantly different between lesioned birds and control birds: percent similarity (p = 0.61, t-test), sequential match (p = 0.08, t-test), and accuracy (p = 0.81, t-test). NCM-lesioned birds specifically imitated the songs of their tutors and no other songs: The average motif-similarity score in NCM-lesioned birds with their tutors (50.7 ± 10.9 ) was much higher than the average score of 36.4 ± 6.0 with 14 unfamiliar adult birds (range 29.7 to 49.0, N = 12, p = 0.0006, t-test, Panel B).
We did not find evidence that song learning happened mainly on the days on which the tutor was present with the juvenile. We tested whether pupils’ songs improved in the absence of direct interactions with the tutors. In 4 birds (3 with lesions and 1 control) that were tutored for only 7 days, we calculated tutor-pupil motif similarity scores both on the first day after tutoring offset (53 dph) and at the end of the experiment (75–77 dph). We found that during this period of isolation, the song motifs in all 4 pupils became more similar to their tutors’ motifs than to motifs in 14 unfamiliar adult birds, panel D and E. Thus, birds could improve songs without the presence of their tutors, implying that they used a memory of tutor song that they acquired within a week after the bilateral lesions in NCM.