The phrase "hollow bones" means that the bones of an animal are not solid but have empty spaces inside them. This is often seen in birds, as their hollow bones help them stay light and fly more easily. Full definition
They share many skeletal features, such as hollow bones, as well as feathers and nest - building behaviors. (sciencemag.org)
Schweitzer hypothesizes that only members of the theropod dinosaur family, which have thin, hollow bones like birds, grew this special layer. (discovermagazine.com)
Hollow bones of Pterasaurs resulted in poor preservation as fossils. (fossilmuseum.net)