It can be tricky to determine exactly why an animal has the features it does, but on this point, it's clear: birds' hollow bones don't make them any lighter. Bird skeletons don't weigh any less than mammal skeletons of the same size. Think about it: thin, hollow bones are more fragile, so they'd need to be much denser to keep from breaking all the time. That density also helps with flying, according to research out of University of Massachusetts Amherst. "Many other studies have shown that as bone density increases, so do bone stiffness and strength," said researcher Elizabeth Dumont. "Maximizing stiffness and strength relative to weight are optimization strategies that are used in the design of strong and stiff but lightweight man-made airframes."
Ask any elementary school student why birds have hollow bones, and they'll have a easy answer for you: it's to help them fly! Gold star, kid. But how do hollow bones really help birds fly? Contrary to popular belief, it's not because it makes them lighter. It's because they need so much oxygen to fly that their lungs actually extend into their bones.