Zebra finch introductory notes become song-like but have different timing and acoustic correlations with the song that follows

Male zebra finches begin their song bouts by repeating a short sound called an introductory note (IN). As INs repeat, their timing and acoustic properties change, similar to pre-movement neural preparatory activity seen in primates and rodents. Hence, INs are thought to reflect motor preparation for starting song. However, INs themselves are vocalizations/movements, like the song that follows, suggesting that INs could be part of the song sequence. To determine how similar songs and INs are, we compared their properties in adult, male zebra finches recorded across 3 years. First, song and INs simultaneously changed in the first year post-hatch; as described earlier, songs became faster and we found an increase of 0.58 +/- 0.07 in mean IN number. Second, as described earlier, IN properties changed from first to last IN. Here, we found that these changes were driven by the last IN becoming more like the first song syllable in duration, amplitude and timing. Third, within each bout, first song syllable timing was correlated with last IN timing in only 18% of birds. Amplitude and mean frequency of the first song syllable was correlated not only with the last IN, but also with the first IN and second song syllable in > 40% of birds. Finally, the reduction in gaps between successive INs was not seen in other types of repeat syllables. Overall, the acoustic similarities between INs and song suggest that INs are part of the song sequence, but timing differences suggest different mechanisms for controlling IN timing. Significance Statement Simple movements are preceded by neural preparatory activity before execution and recent studies have suggested that the complex song sequence of the zebra finch is also preceded by preparatory vocalizations called introductory notes. However, given that introductory notes are also vocalizations, it remains unclear whether their properties reflect similarities to song. Here we found that as introductory notes progress to song, both timing and acoustic properties of introductory notes became more similar to song. In contrast, within bout correlations between introductory notes and song were more prevalent for acoustic features and not timing. These results highlight similarities between introductory notes and song, while also suggesting potential differences in the control of introductory note timing and acoustic properties.