Bird bone. Coloured scanning electron micrograph (SEM) of mineralised bone matrix (compact bone) from a bird bone. This bone is denser than the cancellous (spongy) bone found inside bones. It is similar to compact bone found in mammals and dinosaurs, but different to the compact bone found in reptiles other than dinosaurs. Bird, dinosaur and mammal compact bone is termed fibro-lamellar (appearing fibrous) and is well vascularised, but reptile compact bone is lamellar-zonal (layered) and poorly vascularised. Bird bones also have more air spaces, making the bones lighter, which aids flight.