Fig. 2. Estimated `apparent' flight muscle mechanochemical efficiency plotted against body mass (kg) for 15 bird species during migratory climbing flights (+). The wigeon is left out of the analysis as an outlier. The red regression line is based on the biomechanical power output continuously available (P_ac) calculated using default values for C_D,par ranging from 0.25 to 0.4 (Pennycuick, 1989). The blue regression line is based on P_ac calculated using a value for C_D,par of 0.1. Estimates of efficiency for three species of hummingbirds () are also plotted. Calculations and data sources as in Fig. 1.