Which bones are less dense bird or mammal bones?

Bird bones are less dense than mammal bones.

This is because bird bones are filled with air sacs, which help to reduce their weight and make them more efficient for flying. Mammal bones, on the other hand, are filled with marrow, which is a fatty tissue that helps to store energy and produce blood cells. The lower density of bird bones also makes them more flexible, which is important for birds that need to be able to move their wings quickly and easily.