I have exciting information for you! Bird bones are not entirely hollow, rather they are filled with large air pockets. I find the result both delightful and terrifically unsettling!
To expand upon this: a lot of birds’ bones aren’t hollow in order to make them lighter, but instead so that they can take in more oxygen. This is because when you’re flying through the air at speeds of Fast™, it’s very hard to convince the air around you to flow into your nostrils at speeds that let you get enough to breathe. So, birds evolved hollowed out bones to circumvent this, as it allows them to store air.
I feel like not enough people are aware of the fact that birds are nightmare creatures that have respiratory systems that extend directly into their bones. And the same thing was the case with lots of dinosaurs, light bones let birds fly and let dinosaurs grow huge.
