Fig. 6. Scaling of (A) pectoralis muscle power output and (B) net work per wing stroke with body mass M_b. Data are shown for the Phasianidae (open circles), pigeon and Harris’ hawk during take-off (from Table 5), for hummingbirds (Chai and Millard, 1997; Chai et al., 1997), ‘bees’ (euglossine bees, Euglossa imperialis; M. E. Dillon and R. Dudley, in preparation) and worker bumblebees (Cooper, 1993) during loaded hovering or loaded climbing flights. Scaling of (C) pectoralis myofibrillar power output and (D) net myofibrillar work per wing stroke (W_myo) with body mass. In C and D, data are shown for the Phasianidae, pigeon, small hummingbirds and ‘bees’, calculated by assuming relative myofibrillar volumes of 0.85 (Kaiser and George, 1973; Kiessling, 1977), 0.47 (George and Berger, 1966), 0.50 (Suarez et al., 1991) and 0.55 (Casey and Ellington, 1989), respectively. Relationships that are significantly different from zero are denoted by *(P<0.05) and **(P<0.01).