The bird skeleton is highly adapted for flight. It is extremely lightweight but strong enough to withstand the stresses of taking off, flying, and landing. One key adaptation is the fusing of bones into single ossifications, such as the pygostyle. Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true jaw, instead having a beak, which is far more lightweight. The beaks of many baby birds have a projection called an egg tooth, which facilitates their exit from the amniotic egg, and that falls off once it has done its job.
The vertebral column is divided into five sections of vertebrae:
The neck of a bird is composed of 13–25 cervical vertebrae enabling birds to have increased flexibility. A flexible neck allows many birds with immobile eyes to move their head more productively and center their sight on objects that are close or far in distance. Most birds have about three times as many neck vertebrae than humans which allows for increased stability during fast movements such as flying, landing, and taking-off. The neck plays a role in head-bobbing which is present in at least 8 out of 27 orders of birds, including Columbiformes, Galliformes, and Gruiformes. Head-bobbing is an optokinetic response which stabilizes a birds surroundings as they alternate between a thrust phase and a hold phase. Head-bobbing is synchronous with the feet as the head moves in accordance with the rest of the body. Data from various studies suggest that the main reason for head-bobbing in some birds is for the stabilization of their surroundings, although it is uncertain why some but not all bird orders show beahviors of head-bobbing. Birds are the only vertebrates to have fused collarbones and a keeled breastbone. The keeled sternum serves as a site for muscle attachment used for flight or swimming. Flightless birds, such as ostriches, lack a keeled sternum and have more dense and heavier bones compared to birds that fly. Swimming birds have a wide sternum, walking birds have a long sternum, and flying birds have a sternum that is nearly equal in width and height.
The bones of diving birds are often less hollow than those of non-diving species. Penguins, loons and puffins are without pneumatized bones entirely. Flightless birds, such as ostriches and emus, demonstrate osseous pneumaticity, possessing pneumatized femurs and, in the case of the emu, pneumatized cervical vertebrae.
The chest consists of the furcula (wishbone) and coracoid (collar bone), which, together with the scapula (see below), form the pectoral girdle. The side of the chest is formed by the ribs, which meet at the sternum (mid-line of the chest).