Birds

Birds are endothermic (warm-blooded) vertebrates with feathers. Their anterior limbs are modified as wings for flight, while the posterior pair is adapted for walking, swimming, or perching. Other adaptations related to flight include changes in the skeletal, respiratory, circulatory, reproductive, digestive, and excretory systems. The first birds arose from saurishian dinosaurs during the Jurassic (about 180 million years before present). Today's birds still retain many reptilian characteristics such as similarities in behavior, skull structure, and scales on their beak, legs, and feet. Birds have been incredibly successful (among the chordates, they are outnumbered only by the bony fish).

Birds are divided into two subclasses: the Archaeornithes and the Neornithes. The Archaeornithes, are represented by a single extinct species (Archaeopteryx). The first Archaeopteryx specimen was discovered during the nineteenth century. It was about the size of a crow, had a long reptilian tail, thecodont teeth, and a reptilian skull with no beak. It had three fingers on its wings, each bearing a claw. Because of its small sternum and flexible trunk, it's unlikely that Archaeopteryx was a strong flier. Its characteristics are so reptilian that, were it not for the feathers fossilized with the specimen, it would not be recognizable as a potential ancestor of birds. Protoavis, a smaller animal discovered in 1986, may be more closely allied to today's birds).

All birds other than Archaeopteryx belong to the subclass Neornithes. While most neornithes fly, ratites can not. Examples of living ratites include emus, rheas, ostriches, and penguins. Neornithes that fly are carinates (they have a large carina). The largest living carinate is the Andean condor, with a wing span of about 3 meters. The condor, by no means, represents a limit on size for birds (the giant Teratorn had a wingspan of 8 meters).

Modifications Related to Flight. Examine the bird skeleton figure and image. Note the adaptations in the skeleton and other systems that are directly related to flight. The long bones are thin and slender. Likewise, the skull is thin. Teeth have been lost and replaced by a light beak. Many bones lost their central marrow and are now hollow and porous. Note also the reduction in the number of bones of the wrist. The sternum has broadened into a wide keel to provide a base for attachment of wing musculature. Many bones are fused. Those of the pelvis are fused to the lower vertebrae of the back, while the ribs are joined to the vertebrae and sternum. This consolidation insures that birds don't have to expend muscular energy to hold their lower body straight during flight.

The lungs are directly attached to the ribs and have air sacs that extend into the bones. Both of these attributes raise ventilation efficiency to keep up with their increased metabolic needs. They have no urinary bladder and a short large intestine (to cut back on weight). The cerebral hemispheres and cerebellum show a dramatic increase in size over the reptiles to permit more complex behaviors and coordination needed for flight.

Avian Feathers and Skin. Feathers are cornified epidermal appendages that are probably related to scales. They are used for thermoregulation, communication, and as a flight surface. There are three varieties of feathers: contour feathers, down feathers (plumules), and plumes (hairlike feathers). The main axis of the feather is divided into a hollow calamus that inserts into the animal's skin at a feather follicle and a rachis flattened is attached to the shaft and makes up the main flight surface. The vane is built from numerous barbules. The barbules attach to hooklets on adjacent barbs to hold together the vanelike structure of the feather. If the barbs become separated, the bird zips them back together by preening. Contour feathers that form the flight surfaces are called flight feathers. Down feathers provide excellent insulation because numerous fluffy barbs at the quill tip trap insulating air. Feather development begins when mesodermal tissue in the dermis produces a pimply feather primordium. The feather primordium elongates toward the epidermis, forming a feather follicle. Cells at the base of the follicle then begin to grow forming a feather sheath within which is formed an immature feather. The feather sheath and immature feather together make up a pin feather. Eventually, the feather sheath splits open and releases the barbs, and then the remainder of the feather.

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Clawed Hoatrins	Clawed Hoatrins	Moa