Appendix S4. Difference in power margin between two modern birds with similar body mass.

Flight reconstruction of two modern flying birds with similar BM to show difference in the power margin. Power curves indicating the relationship between Pmec and Vt were modeled for a continuous flapping strategy (i.e., q=1) with CDb calculated according the two approaches used in this study: A, CDb = 0.1; and B, CDb = 0.01 SL / Sb (see Methods). Pav is closely similar as both birds weighted almost the same, and thus the mean value of Pav was drawn (pointed horizontal line). The bar-tailed godwit Limosa lapponica (BM = 367g; CDb = 0.136) shows sufficient power margin to sustain flapping flight in the full-range of Vt, using the aerobic activity from the flight muscles. In contrast, the grey partridge Perdix perdix (BM = 372g; CDb = 0.125) cannot sustain a prolonged flapping flight because the Pav does not attain Pmec. Instead, partridges and other Galliformes fly using short bursts supported by anaerobic activity of the fiber muscles (see Bishop and Butler 2015).