Rexxar's point was about mechanisms of locomotion and body size. While we generally expect birds to have a faster metabolism than mammals because they're small

Rexxar's point was about mechanisms of locomotion and body size. While we generally expect birds to have a faster metabolism than mammals because they're small and because flying takes so much energy, the reverse would probably be the case when comparing ostriches and bats. The ostrich is a 250-pound non-flying bird, and bats are very light flying mammals. In that case, we expect the mammal to have the higher metabolism. I've also wondered if non-flying birds defecate less often, and flying mammals more often. I always assumed that birds emptied out so frequently because it's important to keep your weight down if you're flying, and much less important if you're walking.