We recorded directed song from three zebra finches in a foam-covered room. Recordings were made using a tie-clip microphone (AKG C417) and a Zoom F8 multitrack field recorder (sampling rate of 44.1 kHz). Songs were viewed in Adobe Audition (v. 2015.2), and motifs were selected that did not have competing background noise (i.e. wing fluffs, cage noises, and female calls, etc.) (Fig. 1). Using Adobe Audition, motifs were high-pass filtered with a cutoff frequency of 350 Hz. Consecutive motif renditions were taken when possible, on the assumption that this would maximize the similarity in acoustic fine structure of syllables. Eight renditions of individual syllables were then extracted from eight motif renditions for further preparation to be used as psychophysical stimuli in these experiments. The same eight motif renditions were used for each syllable type, and extracted syllables were given identifiers based on the syllable type (position in the motif A–D) and motif rendition (1–8). Thus, for three zebra finch songs, we had syllables A1–A8, B1–B8, etc.

After individual syllable renditions were extracted, motif stimuli were generated in MATLAB (MathWorks, Natick, MA). Stimulus motifs were created, making two adjustments in order for the stimuli to be appropriate for psychophysical testing. First, inter-syllable silences were fixed for each stimulus motif so that birds could not use differences in inter-syllable silences as a cue. These inter-syllable silences were based on the naturally occurring silences for a single rendition of that male’s motif. Second, each extracted syllable was given a 5 ms cosine rise/fall time. Consistent rise/fall times are necessary to preserve the acoustic features of syllables following inter-syllable intervals of complete silence.

In the psychophysical discrimination experiments, the repeating background stimulus was a motif with syllables in the natural order. For the whole motif rendition set, the background was composed of each syllable from one motif rendition (A1, B1, C1, D1) and each of the seven targets were comprised of each syllable type from a different rendition (e.g. A2, B2, C2, D2). For the other four stimulus sets, the background was again composed of each syllable from one motif rendition (A1, B1, C1, D1), and for each of the seven targets a single syllable was substituted from a different rendition (e.g. A2, B1, C1, D1; A3, B1, C1, D1) (Fig. 1).

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