Birds are different to most mammals in a number of immediately obvious ways, they walk on two legs and have two wings, they have feathers instead of hair and a beak instead of jaws with teeth. The are also similar to us in some ways, they are warm blooded with a normal body temperature of around 40C, several degrees warmer than most mammals, they also have two eyes and two ears, though these are not normally visible. Most birds have little or no sense of smell. Birds have also had to evolve a compact body shape in order to facilitate flying. All the major skeletal differences between birds and their reptilian ancestors are a response to the requirements for flight.
The Basic Skeleton
All birds have the same basic plan, though different life styles have meant that they have evolved different variations on the central theme. Birds such as swans have more vertebrae in their necks than those like parrots who have very short necks. Swans have 25 vertebrae, parrots have 9. Necks are useful for getting your mouth to places you do not really want to take your whole body, like the bottom of a pond. They are also useful for turning your head, when your eyes are fixed deep in sockets, without having to turn your whole body. Most flightless birds, having no need of the large flight muscles, have therefore, no need of the large keel or sternum that flying birds have. The exception to this rule are penguins who effectively fly through the water with their modified wings and still need large muscles to power them.
Bird's inherited their basic skeleton from their reptilian ancestors. The constraints of flight however have meant that birds have had to modify it in several major ways. Flight means lifting the birds weight, so the first major consideration is reduction in weight. The lighter you are the easier it is to fly. The main ways birds have lost weight is through the loss of teeth and the large jaw bones needed to support teeth, the loss of nearly all the tail and reduction of the skull. Though a bird's major limb bones are hollow with internal struts for support, this makes them stronger not lighter; a bird's leg bones for example are often heavier than those of similar sized mammal or reptile.
The flapping of wings to achieve flight requires huge muscles and these muscles need to be solidly attached to the skeleton. They also to generate tremendous stresses in the skeleton when working. A bird gets around the first problem by having a greatly enlarged sternum sometimes called a keel or carina which we call the breast plate. This is missing in certain flightless birds such as the ratites (Emus and Ostriches etc.) It is also missing in Archeopteryx, however scientists believe that in the ratites the sternal keel has been lost as a result of the birds having adopted a flightless life style, whereas in Archeopteryx it had not evolved yet. The second problem is overcome by adding rigidity to the skeleton. The rigidity has been achieved by fusing groups of vertebrae, fusing the two collar bones to make what we call the 'wishbone' and by the addition of special lateral (sideways) growths on the ribs which rest against the next rib back and thus strengthening the whole ribcage. These extensions are called uncinate processes.
A bird's thorax is squat and compact in comparison with most other vertebrates - this brings the operation of both the legs and the wings closer to the centre of gravity allowing them to work more efficiently. This also gives a bird a better balance, important in both flight and bipedal (two legged) locomotion.
To keep their centre of balance when walking birds have evolved to have their equivalent of our thigh held permanently close to the body. The leg does not start to extend out from the body until after the knee joint which is never seen. The backward bending leg joint that you see in bird's legs when they are walking is the equivalent of our ankle. A bird's foot is the equivalent of the tips of our toes. Thus the part of a bird's leg that looks like its shin is actually the equivalent of the arch of our foot.
Information on this page was contributed by EarthLife.
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