A bird's entire skeleton makes up only about 5 percent of its weight
Flight demands a body that is both feather-light and tough enough to survive take-offs and landings. Birds manage it with hollow, strut-braced bones, fused bone clusters and a toothless beak in place of a heavy jaw. The result: a skeleton accounting for roughly one-twentieth of body weight, and a skull about one percent.
Many bird bones are pneumatised, meaning hollow and reinforced inside with criss-crossing struts. Air sacs from the respiratory system often extend into them. Big gliders and soarers tend to have the most hollow bones, while divers have fewer; penguins, loons, puffins and kiwis have none at all. Even flightless ostriches and emus keep some hollow bones, including the emu's neck vertebrae.
Fusion is another trick. Birds generally have fewer bones than other land vertebrates because many are welded together. The last few tail vertebrae form the pygostyle, anchoring the tail feathers that help steer. A block called the synsacrum combines lumbar, sacral and tail vertebrae and joins the hip, carrying the body's weight on the ground. Birds are the only living vertebrates with fused collarbones, the wishbone, and a keeled breastbone that anchors the flight or swimming muscles. Flightless ostriches lack the keel and have denser bones. Overlapping hooks on the ribs, also seen in the tuatara, stiffen the chest, and the heaviest bones sit in the legs, lowering the centre of gravity.
The neck is remarkably flexible. It contains anywhere from 8 to as many as 25 vertebrae in some swans, and most birds have about three times as many neck bones as humans. Since many birds cannot move their eyes, a mobile neck lets them aim their gaze. At least eight of 44 bird orders, including pigeons and chickens, bob their heads, a reflex that steadies their view as they alternate between thrusting forward and holding still, though why only some orders do it is unclear.
The skull tells an evolutionary story. Adult bird skulls resemble the juvenile skulls of their theropod dinosaur ancestors, a pattern called pedomorphosis, having lost teeth and several bones along the way. Made of many small, non-overlapping bones, the skull can flex at joints, known as cranial kinesis, which shapes how birds feed. Many hatchlings carry an egg tooth on the beak to break out of the shell, and it falls away afterwards.
Source: Bird anatomy
