Fig. 7. Breathing frequency in birds scales approximately to M_b^–1/4, and breath duration scales approximately to M_b^1/4 (Frappell et al., 2001; Lindstedt and Calder, 1981). M_b^1/4 scaling of breath duration is indicated schematically by the heavy gray `V' lines on the diagram: larger birds have relatively longer breaths, and smaller birds have relatively shorter breaths. Representative spike recordings of IPC from five species responding to a CO₂ downstep at t=0 (heavy black line) are overlayed on the figure, and are spaced vertically according to body mass of birds in which they were measured. The composite figure shows the general scaling relationship between spike discharge and breath duration. Note that as relative breath duration decreases with decreased body mass, peak chemoreceptor discharge rate and the magnitude of spike frequency adaptation increase. Note also that some IPC were silenced by 6% CO₂; this was idiosyncratic of individual IPC and not species or mass related. On average, larger birds had lower peak chemoreceptor discharge rates, but their longer breaths give IPC ample time to transmit spike information about lung CO₂ changes.