Fig. 3. Ratio of calculated biomechanical P_ac and estimated biomechanical P_min required for forward flapping flight plotted against body mass (kg). Calculations of P_ac assume _O2max = 33M_h^0.88 and E_fm = 0.3M_b^1.137 (see Fig. 2 and text). Estimates of P_min use Pennycuick's computer program 1A (Pennycuick, 1989, version 1.1); default values for C_D,par range from 0.25 to 0.4. Bird morphological data based on the study of Magnan (1922); see text. Coloured lines refer to Appendix B.