What is a bird?

bird (bûrd) n.

  1. Any of the class Aves of warm-blooded, egg-laying, feathered vertebrates with forelimbs modified to form wings.
  2. Such an animal hunted as game.
  3. Such an animal, especially a chicken or turkey, used as food: put the bird in the oven.
  • Bird Characteristics
  • Two-legged (bipedal) vertebrates (animals with a backbone, includes mammals, amphibians, reptiles, and bony fishes).
  • 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.
  • 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
  • Birds evolved from reptiles.
  • 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
  • Scales- Look at birds’ feet.
  • Yolked, polar eggs
  • Nucleated red blood cells. In mammals the red blood cells lack nuclei.
  • Occipital condyle- A single ball-and-socket device by which the skull is articulated with the first neck vertebra. Mammals have two of these.
  • A single middle ear bone: the stapes. Mammals have three.
  • The lower jaws (mandibles) have five or six bones on each side.
  • 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)
  • Evolution of Feathers
  • Derived from scales of some kind
  • Not exactly sure what advantage(s) promoted the evolution of feathers
  • One scenario
  • Slightly elongated and then frayed scales on the trailing edges of the forelimbs possibly enhanced either primitive gliding or parachuting.
  • As gliding abilities improved and steering requirements increased, so did the elaboration of feathers on the wings and the tail.
  • A second scenario
  • Scales became more feather like as temperature regulation devices, particularly as heat shields, that enabled the organism to be more active in hot, sunny environments
  • Modern lizards that live in hot, sunny climates tend to have large scales that reduce heat loads.
  • Featherlike fraying of a scale’s edges would increase its flexibility and effectiveness as a heat shield during the day and insulation at night
  • Reptilian Ancestors of Birds
  • No doubt birds evolved from Mesozoic reptiles, but which reptiles?
  • Thecodonts or Theropods
  • Thecodont Theory
  • Thecodonts were lightly built reptiles in the early Mesozoic era that gave rise to pterosaurs and crocodiles. Some (eg. Longisquama) had elongated scales, that seem like a natural precursor to feathers.
  • Fourteen shared, derived characters unite birds and crocodilian thecodonts.
  • But there is a 90 million year gap between crocodilian thecodonts and Archaeopteryx.
  • Protoavis, a fossil found in western Texas, dating 75 million years before Archaeopteryx, may be a link, but its poor condition makes classification very difficult.
  • Theropod theory
  • Theropod dinosaurs evolved from thecodonts, and lived simultaneously with Archaeopteryx.
  • Both share 23 of 42 specialized skeletal features of the hand, vertebrae, humerus and ulna, pectoral arch, hindlimb and pelvis.
  • Juvenile dinosaurs and birds both have growth plates with a unique cellular structure in their leg bones.
  • Evolution of Avian Flight- The Debate
  • Arboreal Theory vs. Cursorial Theory
  • Arboreal Theory
  • Evolution of flight started with the parachuting and gliding from elevated perches.
  • The extensions of the bones of the forelimb enhanced by elongated (flight) feathers enabled the ancestors of Archaeopteryx to parachute and glide between trees.
  • The favored theory for many years.
  • Cursorial Theory
  • 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.
  • Elongation of the arms and tail would enhance maneuverability and higher velocities of running and jumping.
  • 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.
  • Protowings, increased arboreal habits and gliding would be the next logical steps.
  • Bird Adaptations
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 hearing.