The physics are different. For a gymnast the support is at the hands, so the force felt by the gymnast is a product of his weight and the distance between his s

The physics are different. For a gymnast the support is at the hands, so the force felt by the gymnast is a product of his weight and the distance between his shoulders and hand palms. For a bird I'm guessing that air applies support to the body and wings, so forces are more evenly distributed throughout its length.