Fig. 6. Body-mass-specific work done per step as a function of acceleration. Net work (filled circles) increased as a result of both an increase in positive work (dotted circles) and a decrease in negative work (open circles). For the fastest accelerations, only positive work was performed. All birds are represented by the same symbol for clarity. The linear regression (N=49) is shown for net work (y=0.402x+0.034, r²=0.89, P<0.01).