Fig. 2. (A) The volume of oxygen consumed (V) increases with increasing duration (D) of song bouts (regression equation: V=0.00069+0.00155D; r=0.82; F=125.9; P<0.001). (B) The average cost of singing a song motif (avg_O2 during song minus avg_O2 before song) remains constant for the range of bout duration. Although the slopes of individual linear regressions are slightly positive for all four individuals, they are not significantly different from zero (t-tests on whether the slopes are different from zero yielded P-values of 0.336, 0.21, 0.42 and 0.11 for the four individuals). (C) The silent periods between motifs in a song bout do not explain much of the variance in A. Residuals from A (O₂ volumes x song duration) are plotted over the difference between bout duration and song duration (i.e. the silent periods between song motifs). The regression is not significant (F=2.53; P=0.117), and the slope is not significantly different from zero (t=1.59; P=0.1172). In all three plots, different symbols indicate data contributed by the four individuals.