Bones that breathe

Contrast-enhanced, ventral-dorsal radiograph of an adult, male budgerigar (parakeet-type bird) illustrating the difference in x-ray absorption between marrow-filled and air-filled (pneumatic) bones of the avian skeleton. The ulna (blue arrow) is representative of marrow-filled bones, which are also typical of mammals and reptiles. The marrow cavity is white, with a tissue density of soft tissue. Compare to the humerus (red arrow), where the marrow cavity is black, with the density of air, similar to the lungs. The title of this post is a bit misleading. Pneumatic bones, like the humerus here, do not breathe: they are not involved in gas exchange. That job still rests on the lungs in birds. Pneumatic spaces in bones are extensions of air sacs (anther unique structure in birds) that serve as storage centers for oxygenated air. This reduces the tidal volume and weight of the lungs, and, for high-flying birds, can store oxygen from closer to sea level, which has higher O2 saturation. The humerus, keel, bones at the base of skull, lumbar vertebrae, and sacrum are prototypical pneumatic bones, but the extent of pneumatic bones in birds is variable. One study of 38 species correlated larger body size and activity (high-flying, soaring birds) with greater distribution of pneumatic bones (Canoville et al. BMC Evolutionary Biology, 2019). Pneumatic bones are not always completely air-filled. Pneumatic bones have few trabaculae of bone, primarily in the epiphyses, often spanned by a little hematopoietic and adipose tissue.