What is a bird? bird (bûrd) n. a. Any of the class Aves of warm-blooded, egg-laying, feathered vertebrates with forelimbs modified to form wings. b. Such an animal hunted as game. c. Such an animal, especially a chicken or turkey, used as food: put the bird in the oven. • Bird Characteristics o Two-legged (bipedal) vertebrates (animals with a backbone, includes mammals, amphibians, reptiles, and bony fishes). o Distinguished from other vertebrates by the presence of feathers, a unique modification of the outer skin.  Feathers are dead structures that wear easily and must be replaced regularly, but are essential for both temperature regulation and flight. o All birds have bills.  Bird bills can vary greatly in form and function from species to species, but they are always toothless and are covered with a horny sheath. • Bird Evolution o Birds evolved from reptiles. o Thomas H. Huxley: Birds are “merely glorified reptiles”.  Homology- Similarity in one or more body parts in different species; attributable to descent from a common ancestor • Birds and reptiles both have o Scales- Look at birds’ feet. o Yolked, polar eggs o Nucleated red blood cells. In mammals the red blood cells lack nuclei. o A single middle ear bone: the stapes. Mammals have three. o The lower jaws (mandibles) have five or six bones on each side. o Archaeopteryx lithographica- The Missing Link  Fossil found in Bavaria in 1861 dated at 135 to 155 mya  Clearly showed • Wing bones • Flight feathers • Pairs of feathers attached to each vertebra of the tail  Archaeopteryx was a crow-sized, bipedal “reptile” with a blunt snout and many small, reptilian teeth. • Feathers on both wings and tail• A strong-running terrestrial “bird” that could leap into trees, jump among branches and make short flights. • Capable of gliding, but not long sustained flight. • Had strong, curved claws, like those of perching birds. • Could not launch from the ground because it lacked the principal muscles that lift the wing rapidly in the recovery stroke. • Vanes were asymmetrical, like that of strong fliers.  Immensely important for the theory of evolution. • Found only two years after Darwin published Origin of Species (1859) o Evolution of Avian Flight- The Debate  Arboreal Theory vs. Cursorial Theory • Arboreal Theory o Evolution of flight started with the parachuting and gliding from elevated perches. o The extensions of the bones of the forelimb enhanced by elongated (flight) feathers enabled the ancestors of Archaeopteryx to parachute and glide between trees. o The favored theory for many years. • Cursorial Theory o Forelimbs first elongated because they heightened leaping ability in a small bipedal theropod dinosaurs that ran and jumped to catch insects in its jaws. Extension of forelimbs would help to control and extend its leaps. o Elongation of the arms and tail would enhance maneuverability and higher velocities of running and jumping. o Uses adaptive steps based on trajectory ballistics, rather than the aerodynamics of true flight. Flight would be a logical extension of the first small jumps by this little dinosaur. o Protowings, increased arboreal habits and gliding would be the next logical steps. • Bird Characteristics o Birds are feathered flying machines.  Skeleton is strengthened and reinforced through fusion of bones of the hands, head, pelvis and feet.  Uncinate processes overlap other ribs and so strengthen the walls of the thorax.  The furcula (wishbone) compresses and rebounds like a spring in rhythm to the beat of the wings.  Wings are modified forelimbs, whose sole (almost) purpose is flight.  Fused hand bones support and maneuver the flight feathers.  Arboreal (tree-dwelling) birds have feet that tightly grip branches. An enlarged, keeled sternum houses and anchors the large breast muscles that empower wings.  The pygostyle, made of fused tail vertebrae, supporsta nd controls the tail feathers, which are used for breaking and steering. o Bird physiology accommodates the extreme metabolic demands of flight and temperature regulation.  Red fibers of avian flight muscles have an extraordinary capacity for sustained worka nd can also produce heat by shivering.  Birds maintain high body temps (40° to 44°C) over a wide range of ambient temps.  Circulatory and respiratory systems • Four-chambered heart and efficient, flow-through lungs, which deliver fuel and remove both waste and heat produced by metabolic activities. o Reproduction  Large, richly provisioned external eggs, the most elaborate reproductive cells of any animal.  Requires dedicated parental care.  Most birds form monogamous pairs, though many engage in additional sexual liaisons. o Large well, developed brains 6 to 11 times larger than that of similarly sized reptiles  Bird brains and primate brains exhibit functional lateralization, with left hemispheric dominance associated with learning and innovation in vocal repertoires. o Highly developed neural systems and acute senses mediate feats of communication and navigation.  Birds (esp. song birds) have the greatest sound-producing capabilities of all vertebrates.  Birds can navigate using patterns of the Earth’s magnetism, celestial cues, and perhaps polarized light.  Birds can see into the near-ultraviolet and can hear infrasounds-sounds below the range of human