Prepared by Dennis Paulson
Bear in mind that it is illegal to possess migratory birds or their parts, so this exercise is best done in association with a university or museum. If you are working in a classroom situation, you may also have a variety of types of birds available; this will provide a much better idea of the variations in bird anatomy. If not, a single individual will suffice for the exercise. By dissecting a bird, which means "take it apart," you will be able to look at any part of it you wish.
The major goal of this exercise is for you to learn something, so if you are being supervised, ask questions any time your curiosity is piqued or you're not sure what you're looking at (or for).
By handling it and using your imagination, understand the difference in proportions of the bird with and without its feathers. A bird without feathers is shaped something like a turtle, with compact trunk, long neck, and short tail (uropygium). The neck is highly flexible because of the number of vertebrae and type of vertebral connections, and the bird can easily reach every part of its body with its bill, which is the equivalent of the grasping organ formed by our own opposable thumb and fingers.
The bird with all its feathers presents a rather different appearance, as the wings and tail are then prominent features and the neck is much less obvious in many species. All the small feathers of the body are called contour feathers,and they are arranged in clearly defined tracts, the pterylae (sing. pteryla); the space between them are called apteria (sing. apterium). These details can be seen on a plucked specimen. Note that some feathers are hairlike (filoplumes). The feathers important for flight are large and stiff. The wing and tail coverts cover these feathers and grade into the body contour feathers. Note that the skin is thin, one of the many adaptations to lower the weight for flying. It will be seen again and again that the anatomy of birds is highly modified in many ways as an adaptation to flight.
The head is large, with a prominent toothless bill (also called "beak"). The earliest birds (Archaeopteryx and others) had teeth, but these relatively heavy structures were abandoned evolutionarily in favor of a horny sheath (rhamphotheca) that encases the upper and lower bills (maxilla and mandible). This sheath becomes worn and is continuously replaced, just as the epidermal scales of reptiles become worn, but the latter are shed and replaced all at once at intervals. The external nares (nostrils) vary from absent to prominent.
The eyes are prominent, those of a large owl, for example, about the same size as human eyes. Visual perception is extremely well-developed in this class, and it is obviously advantageous for a fast-moving aerial animal to have good distance vision and depth perception. Look at the eyelids and try to find the nictitating membrane, or "third eyelid," a protective film that can be drawn over the eye from the front. The external auditory meatus (ear opening) is prominent, although covered by feathers. Hearing is also well developed in birds, but the olfactory sense is poor compared with that of other vertebrates.
The trunk is compact, with much fusion of the underlying skeletal elements for structural rigidity. The keel of the sternum, to which the large flight muscles are attached, is obvious on the ventral midline of the breast. The anterior limbs are modified into wings and are much altered from the primitive vertebrate condition characteristic of their reptilian ancestors. The orientation of the upper arm (humerus) and forearm (radius and ulna) is much like that in reptiles, but the wrist and hand are fused, elongated, and directed caudad (tailward). The prominent hand bone is the carpometacarpus. There are only three digits, the first, second, and third. The first projects from the base of the hand and supports the alula, an important structure that prevents stalling during low-speed flight. The second digit forms the tip of the hand, and the third is reduced and cannot be seen externally. The outermost remiges (flight feathers, sing. remex), supported by the hand, are called primaries, the next series in, on the forearm are called the secondaries, and those supported by the upper arm are the tertials. During flight the primaries travel the greatest distance and are very important in propulsion (thrust); the feathers closer to the base of the wing become increasingly instrumental in providing lift. Watch some fairly large birds in flight for an understanding of the mechanics of this adaptation.
The hind limbs, then, are the only ones available for contracting the substrate, and all birds are bipedal. This pair of limbs has rotated 90 degrees from the primitive condition, the original dorsal surface now facing anteriad. The more distal parts of the leg--the lower leg, ankle, and foot--are covered with scales homologous to those of reptiles. The tarsometatarsus ("tarsus") is the prominent part of the leg proximal to the foot and is made up of components from the tarsals and metatarsals. To understand the homologies, be sure you know which these bones are in your own limbs. The fifth digit is lacking from the foot, and the first, or hallux, is directed backwards in most birds as an adaptation for perching. The vent lies beneath the uropygium, and on the top of the uropygium is a prominent papilla, the uropygial gland, which contains the fatty acids, fat, and wax (and possibly vitamin D) the bird uses for preening its feathers. Unlike mammals, which are abundantly supplied with skin glands, birds have only this one.
Pull out different types of feathers and look at their fine details under a dissecting microscope. Disarray them to see how they can be preened (by yourself) back into working order. Manipulate the bill, the neck, the wings, and the tail to see how they work. Check out the tongue when you open the bill and try to envision how it would work to manipulate food items.
The birds available to the whole class should be compared with one another. Note differences in bills, which serve as knives, forks, and spoons, as well as all-purpose manipulating organs; they are adaptively modified depending on the diet of the species or group. Words that describe different bill types include long, short, hooked, crossed, compressed, depressed, stout, terete, straight, recurved, decurved, bent, swollen, acute, obtuse, chisel-like, toothed, serrate, spatulate, notched, conical, lamellate. Nostrils can be round, oval, linear, in a soft cere, or partially covered by feathers or a soft operculum. Compare the size of the gape, an indication of the size of potential prey. Tongues vary too: woodpeckers have an especially long one, with a barbed tip, and they probe into crevices and extract insects with it. Other birds' tongues vary from long, slender, and cornified to short and fleshy.
Also compare the feet of your sample of bird types; they are modified for both locomotion and prey capture. There are scales on the tarsi of most birds; they can be scutellate, reticulate, scutellate-reticulate, booted, scutellate-booted, or spurred. The tarsi can be terete in cross-section or ridged behind. The toes can all be on the same level (hallux incumbent) or with the hallux elevated above the front toes. The nails can be acute or obtuse, straight or curved, long or short, flattened, and/or pectinate. The arrangement of the toes can be anisodactyl, zygodactyl, syndactyl, or pamprodactyl. More obvious modifications of the toes include raptorial, palmate, semipalmate, totipalmate, and lobate. See what you can find among the lab birds.
Compare wing and tail shapes--especially the shape of the tips--which vary with birds' flight styles, and note the relative size of the wings (in comparison with the body bulk) to get some idea of the bird's flight abilities. Wings can be long, short, narrow, broad, pointed, rounded, flat, concave, and/or spurred (spur at bend of wing). Tails can be long, short, square, rounded, graduated, pointed, emarginate, or forked, or, of course, anything in between the modes that are most similar. The structure of the feathers themselves varies in different species and groups. Look for crests, head tufts, and facial discs. Look for areas where feathers are reduced, modified, or absent, especially on the head. Look at differences in eye size and placement, and at the ears of an owl if available. In owls, the different size and placement of the two ears allows precise localizations of sounds, of great importance to these auditory hunters. Compare uropygial glands in water and land birds; a few species lack them entirely.
Now dissect your bird. Make a midventral slit from the sternum to just before the vent with scissors; do not cut through the vent. Part the skin and superficial muscles of the abdomen, then continue the cut through the ribs of the bird's left side all the way to the base of the wing. Lay back the sternum so you can see the organs in the abdominal cavity. Peel the skin back from the breast so you can see the large breast muscles. The pectoralis major, which depresses the wing for the powerful downstroke during flight, covers the entire sternum. Cut through it carefully until you encounter the smaller muscles beneath. These muscles--primarily the supracoracoideus--run through a notch in the pectoral girdle and attach on the dorsal surface of the humerus, elevating it for the upstroke. The pectoralis and supracoracoideus, as might be expected, are the largest muscles in most birds.
The digestive system is the next to examine. The intestine is prominent at the posterior end of the abdominal cavity. Carefully pull it out until you can see its posterior end, then carefully follow it forward. From rear to front, note the expanded cloaca, then the rather short large intestine, easily distinguished from the small intestine anterior to it by the pair of intestinal caeca between them. The caeca are rudimentary in most birds but are quite long in those that must digest cellulose from the leaves they eat.
The small intestine is quite long, as in most vertebrates, and clearly distinguished from the muscular gizzard at its anterior end. The gizzard functions both to secrete digestive enzymes and to grind up food, and those species that swallow hard-to-digest items have the most muscular gizzards. Many seed-eating birds (e.g., pigeons and gallinaceous birds) swallow small pieces of gravel that enhance this grinding process. Anterior to the gizzard is the anterior stomach, or proventriculus, a softer, less muscular organ, and anterior to that is the esophagus. In some groups of birds there is a prominent bilobed crop where the esophagus and proventriculus meet; a large meal is stored here before being transferred to the stomach. Look for the rather long, flat, yellowish pancreas in the anteriormost loop of the small intestine and the small, round, reddish spleen dorsal to the stomach.
In ventral view, the prominent liver (with two lobes) covers much of the chest cavity. Pull it out to expose the muscular heart (medial) and paired lungs (lateral). Try to find some of the air sacs associated with the lungs; most may be deflated. Cut into the liver, heart, and lungs to see how different they are structurally. Note the gall bladder, a greenish organ present only in some bird groups; it is especially well-developed in woodpeckers. Cut through the gizzard wall to see the digestive surface within, then cut through it and remove the digestive tract from the body, leaving the posterior end attached.
Along the dorsal wall of the body cavity lie the large, lobed kidneys of the urinary tract. A thin ureter extends from the inner side of each kidney to the cloaca. The gonads lie at the anterior ends of the kidneys. Males have paired bean-shaped (usually whitish) testes, females a single irregularly shaped ovary on the left kidney (some birds, especially hawks, have ovaries on both sides). In immatures, which have small gonads, the small, flat, yellowish adrenal glands can be seen at the anterior ends of the kidneys. In the male the vas deferens (called a seminal vesicle at its posterior end) passes from each testis back to the cloaca. In the female, the oviduct extends from the vicinity of the ovary to the cloaca. Eggs are shed into the abdominal cavity and picked up by the funnel-shaped ostium at the anterior end of the ovary. In the breeding season, the testes enlarge considerably and the ovarian follicles develop into quite conspicuous eggs.
Finally, carefully cut the skin from the breast up to the side of the bill to see the soft esophagus and the bony-ringed trachea running along the neck. Two bronchi branch from the trachea to the lungs, and at that junction is the syrinx. Vibrations in the bony ridges and membranes of the syrinx produce the complex vocalizations that characterize different species of birds. Cut one corner of the mouth and pull the bill away to be able to see details of the tongue and mouth. The brain of a freshly dead bird is worth examining but deteriorates quickly in birds that are frozen.