full information about birds

Birds have many bones that are hollow (pneumatized) 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.[1] The bones of diving birds are often less hollow than those of non-diving species. Penguins, loons,[2] and puffins are without pneumatized bones entirely.[3][4] Flightless birds, such as ostriches and emus, have pneumatized femurs[5] and, in the case of the emu, pneumatized cervical vertebrae.[6]

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.

Collage of bird anatomical illustrations with the different vertebral sections color-coded across various species. The species included are as follows: Top Row (left to right) Struthio camelus and Sagittarius serpentarius (formerly Gypogeranus serpentarius) Bottom Row (left to right) Megascops choliba decussatus (formerly known as Strix decussata) and Falco rusticolus islandus (formerly Falco islandus).
Sections of the Vertebral Column in Anatomical Bird Diagrams
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 (Cygninae) and other long-necked birds. All cervical vertebrae have ribs attached except the first one. This vertebra (C1) is called the atlas which articulates with the occipital condyles of the skull and lacks the foramen typical of most vertebrae.[7] 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.[8] 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.[9] 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.[10] Head-bobbing is an optokinetic response which stabilizes a bird's surroundings as it alternates between a thrust phase and a hold phase.[11] Head-bobbing is synchronous with the feet as the head moves in accordance with the rest of the body.[11] 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.[12]

Thoracic vertebrae
The thoracic vertebrae number between 5 and 10, 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.[7] Anterior thoracic vertebrae are fused in many birds and articulate with the notarium of the pectoral girdle.[13]

Diagram of a general bird pelvic girdle skeleton including the lower vertebral column sections. The diagram includes the synsacrum, caudal, and pygostyle vertebrae. Note that the caudal vertebrae (5–10) are not fused in this diagram but can be in certain species.
Diagram of a general bird pelvic girdle skeleton including the lower vertebral column sections. Note that the caudal vertebrae (5–10) are not fused in this diagram but can be in certain species.
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.[14] When not in flight, this structure provides the main support for the rest of the body.[7] Similar to the sacrum of mammals, the synsacrum lacks the distinct disc shape of cervical and thoracic vertebrae.[15]

Caudal vertebrae
The free vertebrae immediately following the fused sacro-caudal vertebrae of the synsacrum are known as the caudal vertebrae. Birds have between 5 and 8 free caudal vertebrae.[7] The caudal vertebrae provide structure to the tails of vertebrates and are homologous to the coccyx found in mammals lacking tails.[16]

Pygostyle
In birds, the last 5 to 6 caudal vertebrae are fused to form the pygostyle.[14] Some sources note that up to 10 caudal vertebrae may make up this fused structure. This structure provides an attachment point for tail feathers that aid in control of flight.[7]

Highlighted in red is an intact keeled sternum of a dissected pigeon. In flying birds the sternum is enlarged for increased muscle attachment.
Scapular girdle
Birds are the only living vertebrates to have fused collarbones and a keeled breastbone.[17] The keeled sternum serves as an attachment site for the muscles used in flying or swimming.[17] Flightless birds, such as ostriches, lack a keeled sternum and have denser and heavier bones compared to birds that fly.[18] 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.[19] The chest consists of the furcula (wishbone) and coracoid (collar bone) which, together with the scapula, form the pectoral girdle; the side of the chest is formed by the ribs, which meet at the sternum (mid-line of the chest).[7]

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 (Sphenodon).

Skull

The typical cranial anatomy of a bird. Pmx= premaxilla, M= maxilla, D= dentary, V= vomer, Pal= palatine, Pt= Pterygoid, Lc= Lacrimal
The skull consists of five major bones: the frontal (top of head), parietal (back of head), premaxillary and nasal (top beak), and the mandible (bottom beak). 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.[20] 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.[21][22] The palate structures have also become greatly altered with changes, mostly reductions, seen in the ptyergoid, palatine, and jugal bones. A reduction in the adductor chambers has also occurred [22] 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[20] and paleontological [23] 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".[22] The premaxilla is also known to play a large role in feeding behaviours in fish.[24][25]

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 (including the ancestors of modern birds)[26][27][28] have akinetic (non-kinetic) skulls.[29][30] For this reason it has been argued that the pedomorphic bird beak can be seen as an evolutionary innovation.[22]

Birds have a diapsid skull, as in reptiles, with a pre-lachrymal fossa (present in some reptiles). The skull has a single occipital condyle.[31]

Appendicular skeleton

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The shoulder consists of the scapula (shoulder blade), coracoid, and humerus (upper arm). The humerus joins the radius and ulna (forearm) 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 (top of the hip), ischium (sides of hip), and pubis (front of the hip). These are fused into one (the innominate bone). Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at the acetabulum (hip socket) 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 (shin) and fibula (side of lower leg). 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
Main article: Bird wings
Feet
Main article: Bird feet and legs

Four types of bird feet
(right foot diagrams)
Birds' feet are classified as anisodactyl, zygodactyl, heterodactyl, syndactyl or pamprodactyl.[32] 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 (the inner and middle forward-pointing toes), or three toes, are fused together, as in the belted kingfisher Ceryle alcyon. This is characteristic of Coraciiformes (kingfishers, bee-eaters, rollers, etc.).

Zygodactyl (from Greek ζυγον, a yoke) feet have two toes facing forward (digits two and three) and two back (digits one and four). This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots, woodpeckers (including flickers), cuckoos (including roadrunners), and some owls. Zygodactyl tracks have been found dating to 120–110 Ma (early Cretaceous), 50 million years before the first identified zygodactyl fossils.[33]

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 (Apodidae).