Bird anatomy
Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has led to evolution of a specially adapted digestive system.
Skeletal system
Birds have many bones that are hollow with criss-crossing struts or trusses for structural strength. The number of hollow bones varies among species, though large gliding and soaring birds tend to have the most. Respiratory air sacs often form air pockets within the semi-hollow bones of the bird's skeleton. The bones of diving birds are often less hollow than those of non-diving species. Penguins, loons, puffins, and kiwis are without pneumatized bones entirely. Flightless birds, such as ostriches and emus, have pneumatized femurs and, in the case of the emu, pneumatized cervical vertebrae.In most birds, non-pneumatized bones are filled with bone marrow.
Axial skeleton
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 and instead have 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. It falls off once the egg has been penetrated.File:Diversity of Bird Vertebrae.png|left|thumb|271x271px|Collage of bird anatomical illustrations with the different vertebral sections color-coded across various species. The species included are as follows: Top row Struthio camelus and Sagittarius serpentarius Bottom row Megascops choliba decussatus and Falco rusticolus islandus.
| Color | Vertebral section |
| Pink | Cervical vertebrae |
| Orange | Thoracic/dorsal vertebrae |
| Yellow | Synsacrum |
| Green | Caudal vertebrae |
| Blue | Pygostyle |
Vertebral column
The vertebral column is divided into five sections of vertebrae:Cervical vertebrae
The cervical vertebrae provide structural support to the neck and number between 8 and as many as 25 vertebrae in certain swan species and other long-necked birds. All cervical vertebrae have transverse processes attached except the first one. This vertebra is called the atlas which articulates with the occipital condyles of the skull and lacks the foramen typical of most vertebrae. The neck of a bird is composed of many 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 as 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 eight out of 44 orders of birds, including Columbiformes, Galliformes, and Gruiformes. Head-bobbing is an optokinetic response which stabilizes a bird's surroundings as it alternates 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 head-bob.Thoracic vertebrae
The thoracic vertebrae number between five and ten, and the first thoracic vertebra is distinguishable due to the fusion of its attached rib to the sternum while the ribs of cervical vertebrae are free. Anterior thoracic vertebrae are fused in many birds and articulate with the notarium of the pectoral girdle.Synsacrum
The synsacrum consists of one thoracic, six lumbar, two sacral, and five sacro-caudal vertebrae fused into one ossified structure that then fuse with the ilium. When not in flight, this structure provides the main support for the rest of the body. Similar to the sacrum of mammals, the synsacrum lacks the distinct disc shape of cervical and thoracic vertebrae.Caudal vertebrae
The free vertebrae immediately following the fused sacro-caudal vertebrae of the synsacrum are known as the caudal vertebrae. Birds have between five and eight free caudal vertebrae. The caudal vertebrae provide structure to the tails of vertebrates and are homologous to the coccyx found in mammals lacking tails.Pygostyle
In birds, the last four caudal vertebrae are fused to form the pygostyle. Some sources note that up to ten caudal vertebrae may make up this fused structure. This structure provides an attachment point for tail feathers that aid in control of flight.Scapular girdle
Birds are the only living vertebrates to have fused collarbones and a keeled breastbone. The keeled sternum serves as an attachment site for the muscles used in flying or swimming. Flightless birds, such as ostriches, lack a keeled sternum and have denser 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 chest consists of the furcula and coracoid which, together with the scapula, form the pectoral girdle; the side of the chest is formed by the ribs, which meet at the sternum.Ribs
Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them. This feature is also found in the tuatara.Skull
The skull consists of five major bones: the frontal, parietal, premaxillary and nasal, and the mandible. The skull of a normal bird usually weighs about 1% of the bird's total body weight. The eye occupies a considerable amount of the skull and is surrounded by a sclerotic eye-ring, a ring of tiny bones. This characteristic is also seen in their reptile cousins.Broadly speaking, avian skulls consist of many small, non-overlapping bones. Pedomorphosis, maintenance of the ancestral state in adults, is thought to have facilitated the evolution of the avian skull. In essence, adult bird skulls will resemble the juvenile form of their theropod dinosaur ancestors. As the avian lineage has progressed and as pedomorphosis has occurred, they have lost the postorbital bone behind the eye, the ectopterygoid at the back of the palate, and teeth. The palate structures have also become greatly altered with changes, mostly reductions, seen in the pterygoid, palatine, and jugal bones. A reduction in the adductor chambers has also occurred. These are all conditions seen in the juvenile form of their ancestors. The premaxillary bone has also hypertrophied to form the beak while the maxilla has become diminished, as suggested by both developmental and paleontological studies. This expansion into the beak has occurred in tandem with the loss of a functional hand and the developmental of a point at the front of the beak that resembles a "finger". The premaxilla is also known to play a large role in feeding behaviours in fish.
The structure of the avian skull has important implications for their feeding behaviours. Birds show independent movement of the skull bones known as cranial kinesis. Cranial kinesis in birds occurs in several forms, but all of the different varieties are all made possible by the anatomy of the skull. Animals with large, overlapping bones have akinetic skulls. For this reason it has been argued that the pedomorphic bird beak can be seen as an evolutionary innovation.
Birds have a diapsid skull, as in reptiles, with a pre-lachrymal fossa. The skull has a single occipital condyle.
Appendicular skeleton
The shoulder consists of the scapula, coracoid, and humerus. The humerus joins the radius and ulna to form the elbow. The carpus and metacarpus form the "wrist" and "hand" of the bird, and the digits are fused together. The bones in the wing are extremely light so that the bird can fly more easily.The hips consist of the pelvis, which includes three major bones: the ilium, ischium, and pubis. These are fused into one. Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at the acetabulum and articulate with the femur, which is the first bone of the hind limb.
The upper leg consists of the femur. At the knee joint, the femur connects to the tibiotarsus and fibula. The tarsometatarsus forms the upper part of the foot, digits make up the toes. The leg bones of birds are the heaviest, contributing to a low center of gravity, which aids in flight. A bird's skeleton accounts for only about 5% of its total body weight.
They have a greatly elongate tetradiate pelvis, similar to some reptiles. The hind limb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk vertebrae as well as fusion with the pectoral girdle.
Wings
Feet
are classified as anisodactyl, zygodactyl, heterodactyl, syndactyl or pamprodactyl. Anisodactyl is the most common arrangement of digits in birds, with three toes forward and one back. This is common in songbirds and other perching birds, as well as hunting birds like eagles, hawks, and falcons.Syndactyly, as it occurs in birds, is like anisodactyly, except that the second and third toes, or three toes, are fused together, as in the belted kingfisher Ceryle alcyon. This is characteristic of Coraciiformes.
Zygodactyl feet have two toes facing forward and two back. This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots, woodpeckers, cuckoos, and some owls. Zygodactyl tracks have been found dating to 120–110 Ma, 50 million years before the first identified zygodactyl fossils.
Heterodactyly is like zygodactyly, except that digits three and four point forward and digits one and two point back. This is found only in trogons, while pamprodactyl is an arrangement in which all four toes may point forward, or birds may rotate the outer two toes backward. It is a characteristic of swifts.