A widely cited review of the digestive efficiency of birds concluded that the apparent Metabolisable Energy Coefficient (*MEC), a measure of the efficiency of energy assimilation, did not vary between bird orders. To test whether this result was due to the variation between the experimental methods of the studies chosen for the review, the *MEC was measured for four species (two galliforms and two columbiforms) fed a variety of seeds. The *MEC of the birds did not vary in any consistent manner when fed identical diets, thus supporting the review's result. However, the *MEC of each species varied consistently with change in diet. These findings can be explained by the statistical power of the *MEC comparisons. Comparisons of *MEC between bird species are less powerful than comparisons between diets.