“Ding” Darling National Wildlife Refuge reported that the great blue heron stands about 4 feet tall with a wingspan that can be up to 7 feet. The term for this characteristic is pneumatized, which is defined as having spaces of air. Large gliding birds tend to have more pneumatized bones, while diving birds have less. In addition to a lightweight skeleton, feathers, air sacs and overall body design contribute to weight reduction. Birds also have an interesting respiratory system: air flows through their lungs in a mostly one-way direction with the help of air sacs, allowing fresh air to pass through their lungs during inhalation and exhalation.