Gap duration plotted against inter-syllable frequency ratio. Blue line: lower-bound regression line. Red diamonds: syllables produced in response to territorial playbacks. Black triangles: syllables produced in spontaneous song (both contexts together n = 6,910 syllables of 16 subjects). Blue crosses: data points used to calculate the lower-bound regression line assessed by the equal samples per bin method . Vocal gap deviation is measured as the minimum orthogonal distance of each data point to the lower-bound regression line; an example is shown for one data point (black dotted line). Minimum area convex polygon is given in red for reactive and in grey for spontaneous singing. For statistics see text.