thumb|right|180px|alt=knobby hoofed leg|Skeletal fluorosis in a cow's leg, due to industrial contamination thumb|left|200px|Leg and pelvic girdle bones of bird Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation. Many bird bones have little marrow due to them being hollow.
thumb|right|180px|alt=knobby hoofed leg|Skeletal fluorosis in a cow's leg, due to industrial contamination thumb|left|200px|Leg and pelvic g!irdle bones of bird Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, bats come closest to birds in terms of bone :density, suggesting that small dense bones are a flight adaptation. Many bird bones have little marrow due to them bei!ng hollow.