Class Aves (the birds)

Birds are amniotes who are from the synapsid line of reptilian evolution. They evolved from archosaurian ancestor. Nearest living relatives are the crocodiles (both share a thecodont ancestor). Birds are essentially advanced flying dinosaurs. By late Triassic or early Jurassic there were two lines of reptiles from thecodonts that could fly.

Flying dinosaurs Ptereosaurs included animals with wings of large membranes of skin between the body and enormous elongated arms and fourth fingers. Wingspread up to 25 ft. Died out. Other line developed feathered wings (derived from scales). In time became sufficiently different from others to be placed in a separate class, the Aves. (Some biologists believe that birds don't deserve sep. class. Should be placed in Reptilia as surviving dinosaurs. Others would erect a new class with both birds and dinosaurs.

Archaeopteryx. Ancestral bird is Archaeopteryx. From middle Jurassic. Had many reptilian characteristics; e.g. long jointed tail (only few vertebrae in modern birds) and teeth (beak in modern birds).

Characteristics of birds are essentially adaptations for flight. A necessary correlate of flight is endothermy. So see adaptations to improve endothermy and optimize energetic efficiency as way of enhancing flight efficiency. Bird flight adaptations are concentrated on increasing power without increasing weight.

Characteristics Feathers are modified from reptilian scales. Lightweight, but tough and have high tensile strength. Three types:

Contour feathers Contour feathers of calamus (quill), rachis (shaft) and barb with barbules (form webbed vane). Give bird characteristic body form and are feathers of flight.

Down feathers Down feathers have no hooks in barbs and are soft. They provide insulation in young birds and aquatic species.

Filoplume feathers Filoplume feathers have a hairlike rachis with a tuft of short barbs at the tip. They are degenerate feathers, annoying when plucking a chicken. Sense organs to control other feathers is a possibility because richly supplied with nervous tissue. Birds usually molt because feathers, like hair are dead.

Wings Highly modified forelimbs. Wing and tail feathers greatly increase the surface area of the animal and add relatively little weight. The bones of the wings are the same bones found in our limbs: the humerus, the radius, the ulna, etc..

Skeletal adaptations Skeleton is highly adapted for flight. Light weight bones, many hollow with air sacs which project from the enlarged lungs.

Skeletal adaptations Bipedal because forelimbs are wings. Sternum has keel which provides increased surface area for flight muscles. Beak from anterior skull bones. Fusion of vertebrae (except neck) to pelvic girdle --> stiff, light framework.

Muscular system Specialized for flight. Muscle which raises the wing is normally a back muscle. It is located on the breast and anchored to the keel of the sternum. Raises the wing by attaching to top of wing by tendon so that the action is a pulley. Location is better for flight because it increases the aerodynamic stability of birds.

Digestion Rapid and efficient to provide energy necessary for flight. Many have crop (storage organ and some crops used for "milk" production). Gizzard ("pupick") is part of the stomach for grinding food (no teeth).

Circulatory system The system is similar to mammals. There is a four chambered heart Very efficient!

Respiratory system The system is unique. Lungs attached to body wall so can't expand as ours do. Have air capillaries instead of alveoli as we do. Have anterior and posterior air sacs.

Respiratory system When breathe in (inspiration) most (75%) of air goes directly into posterior air sacs, rest to lungs. When breathe out (expiration) posterior air goes to lungs and air that was in the anterior air sacs moves to the outside. Get oxygen and CO2 exchange in lungs and in air sacs and on both inspiration and expiration. One way system.

Respiratory system We are not so efficient. Air goes to alveoli in our lungs only on inspiration. Sizeable amount remains there at expiration and mixes with "good air" at next inspiration. Not very good. Birds use system to cool as well as ventilate.

Excretory system Excrete uric acid (solid crystals). Permits retention of water which is often a problem for birds. Mixed uric acid solution and fecal waste --> guano. High in Potassium and nitrogen and valuable as fertilizer.

Brain Cerebral cortex not developed (thin and unfissured). Instead core of cerebrum is well developed (corpus striatum). Use for complicated behavioral activities, but these are largely pre-programmed (instinctive). Evolved from reptile which had no neocortex.