I know it’s not because of less weight, IIRC some birds are even heavier than other creatures of their size. I remember hearing something like that it lets them breathe better so they can get the great amount of oxygen they need to sustain flight. But how does that work? I just don’t understand.
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They have air sacs in their bones which can take in air when they respire helping them get and use more oxygen compared to animals without such air sacs. It’s as if they have lungs incorporated into their bones, or maybe it’s more correct to say their lung system extends into their bones.
The answer is lower density. The hollow bones make the long bones particularly the breast bone and wing bones very strong in bending but light so the bird can put more mass budget to muscle’s and feathers.
It’s also why birds lay eggs because it’s less weight to carry around…
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