Aves: More on Morphology

Click on either picture to view an enlargement.

As birds go, the domestic chicken is hardly built for high-performance flight. Yet even in the chicken skeleton shown here, many of the flight adaptations found in most birds can easily be seen. The sternum, or breastbone, bears a prominent keel where the flight muscles attach. (Note that Archaeopt eryx and several other early fossil birds lacked this sternal keel, and the flightless ratite birds -- such as the ostrich, emu, and rhea -- also lack the keel.)

The furcula, a fused clavicle (collarbone), serves as a brace during the flight stroke; it's visible in the pictures above as a large Y-shaped bone ahead of the sternum. The clavicle is also found in non-avian dromaeosaurian dinosaurs, and was probably co-opted in function from the dromaeosaurian function of providing a brace for the shoulder girdle while holding prey. Crucial for bird flight is a canal formed by the articulation of the humerus (forewing bone), the scapula (shoulder blade), and the coracoid (bone connecting the sternum itself to the humerus). Through this canal, the foramen triosseum or triosseal canal, runs the tendon of the supracoracoideus muscle, which attaches to the sternum and the dorsal side of the humerus, and lifts the wing upwards in flight. The powerful downstroke of the wing is powered by the large pectoralis muscles, which also attach to the sternal keel.

A number of other features of birds have the effect of reducing weight or otherwise contributing to flight capabilities. Birds have highly shortened tails, which effectively reduce the bird's weight (compare Archaeopteryx, which had a long bony tail). Living birds lack teeth, and the jaws are covered by a horny beak. Bird bones are hollow and filled with extensions of the body's air sacs, which are extensions of the lungs that allow for increased respiratory efficiency. The hollow bones are thin-walled, but actually quite resistant to stress from bending -- the stresses involved in flight. The eyes are large and the visual cortex of the brain is well-developed. Other anatomical features of birds that are probably connected with flight are not visible in the skeleton. Birds lack a bladder, and they excrete uric acid in the form of solid white crystals, rather than urea in a water solution.

But the most obvious anatomical feature of birds is, of course, their feathers. Feathers are highly modified scales, and are important in several ways. Soft down feathers trap still air close to the surface of the body, thermally insulating a bird (or a human wearing a down jacket). Feathers are also crucial for flight; contour feathers on the body establish the smooth, streamlined contour of a bird's body, and the enlarged flight feathers form the aerodynamic surfaces of the wings and tail. Some birds, such as the flycatchers, have bristlelike feathers around the mouth that assist in trapping insects in flight.
Finally, feathers are important in bird behavior: the brightly colored feathers of the males of many species, such as the finch pictured here, are used as signals during courtship and mating. Bird feathers used in mate attraction may form huge crests, ruffs, or tails: the male peacock tail is a case in point. On the other hand, the females of many species with brightly colored males have rather drab feathers acting as camouflage.

For a short lesson on external bird anatomy, try this tutorial.