What are the biggest differences in the skeletons of mammals/birds/mammals?
If this is just limited to skeletal differences: Bird bones are hollow and often filled with air sacks that are part of their respiratory system. Mammal bones are heavier and mostly solid. The hands of birds have lost most of their fingers and been modified for flight, though differently from a bat’s wig. Bat’s retain five fingers while most birds only have one finger with the others remaining as vestigial. There is a lot of variation though as some bird families have fingers modified into spikes or clubs and the hoatzin retains clawed fingers for climbing as a chick and juvenile. Really, the bird skeleton is hugely specialized for weight reduction.
