A lighter skeleton lets birds take flight while using less energy (fewer calories). Since evolution favors efficiency when it comes to food, it's a good explanation of why birds have hollow bones.
However, hollow bone construction gives us only a partial understanding of how a bird skeleton is designed for flight. Despite a bird skeleton's delicate — dainty, even — appearance, bird bones are also quite dense and heavy, says a 2010 study published in Proceedings of the Royal Society B.
In the study, when comparing like-sized bat and rodent bones, the overall density of the bird bones was higher. Bird bones are not only hollow to achieve lightness, but they’re dense for additional strength, so their frames can withstand the stresses of flight.
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