Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180


- Presence of feathers separates birds from all other living vertebrates. Feathers are associated with flight - the major characteristic feature of birds is the capacity for flight.

- Not all birds are capable of flight, but flightless birds are evolutionarily derived from flying ancestors.

- The entire anatomy and physiology of birds is adapted for efficient flight, so most birds are morphologically very similar because they are constrained by the demands of flight. Differences used to separate different taxonomic Orders of birds would only separate families in other vertebrate groups.

- The 2 major requirements for flight (includes insects, birds, bats, airplanes) are high power and low weight. Birds meet these requirements by several adaptations of their anatomy and physiology.


  • 1) Hollow, strutted, or spongy bone. Many bones are pneumatic (contain air sacs within hollow insides).
  • 2) Fusion of many bones, including vertebrae appendages, and girdles - also provides rigidity to skeleton that is necessary for flight.
  • 3) Reduction in some bones (e.g., tail) and loss of teeth
  • 4) Feathers - lightweight yet well-adapted for aerodynamics and insulation.
  • 5) Urogenital Adaptations:
    • a. No urinary bladder or urethra - kidneys produce mainly uric acid, excreted as semisolid paste so this also contributes to water conservation.
    • b. All lay eggs (no live-bearers) - only vertebrate Class for which this is true.
    • c. Only left ovary persists in most adult females
    • d. sex organs in both sexes are enlarged and functional only during the breeding season; when breeding is complete, gonads regress.


    • 1) Endothermic Homeotherms - maintain highest Tb of any vertebrate, 40 - 43oC (about 37-38oC for mammals); since most aerobic biochemical reactions are temperature dependent, a high Tb --> fast reaction rates --> high and rapid sustainable power production. Birds with the highest aerobic MR/size of any vertebrate.
    • 2) Efficient digestion to acquire fuel to power metabolism
    • 3) Very large breast muscle (pectoralis)
      • - powers downstroke of wing. Makes up about 15-25% (up to 40%) of total body weight.
      • - keeled sternum for attachment of high volume of flight muscle, mostly supracoracoideus which powers upstroke. Absent in ratites (flightless) and Archaeopteryx.
    • 4) 4-chambered heart for efficient circulation and rapid blood delivery
    • 5) Large and complicated respiratory system- very efficient at extracting 02 from inspired air.


    • 1) Large eyes and optic lobes
    • 2) Large cerebellum for optimal muscle coordination

    ** FLIGHT **

    I. Wings are modified forelimbs with unusual articulating surfaces which allow movements necessary for flight. Elongated and fused hand and finger bones for feather attachment.
    II. PHYSICAL PRINCIPLES - very complicated, so just give the basics

    • 1) Lift = aerodynamic phenomenon caused by differential pressures on opposite sides of an airfoil (structure with convex top and flat or slightly concave bottom that tapers posteriorly)
      • - Bird wings are airfoils. They work because as air passes over the wing, it must travel further over the upper surface than the lower surface. Thus, it must move faster on top than on bottom. The faster the air moves the lower the air pressure. Since the bottom has more pressure than the top, you get lift. (SEE HANDOUT)
      • - The angle of the wing (airfoil) is important. A steeper angle relative to air flow increased distance incr. air speed decr. pressure incr. lift. But, too steep an angle produces turbulence as air flow separates from the upper surface stalling
      • - Wing slot can direct air flow over upper surface at a steeper angle. Alula = feather attached to 1st digit (thumb), moves independently from rest of wing so can act as wing slot and incr. lift.

    • 2) Drag = resistance to movement produced by the medium through which an object is moving.
      Airfoil shape produces the most positive lift-to-drag ratio so minimizes power required for flight. (SEE HANDOUT) However, flight is still a very power-intensive process.
    • 3) Thrust = forward motion produced by an airfoil (mainly the primaries) moving vertically rather than horizontally through air. (SEE HANDOUT)
    • 4) Aspect Ratio = Wing Length : Wing Width
      • - Long, narrow wings have a high aspect ratio. A high aspect ratio produces high lift-to-drag ratios by minimizing vortex turbulence (a form of drag) at wing tips.
      • - Thus, the higher aspect ratio, the less power needed to sustain flight. Albatross wings may have aspect ratio of 18 : 1.

    • 5) Wing-loading = Body weight / Wing area
      • - The lower this ratio, the less power needed to sustain flight.

    • 1) Body Size - Mass increases with L3, surface incr. with L2. This means that if a bird were enlarged 2X, its wing area would incr. by 4X, but its mass by 8X.
      • - Large, heavy birds have proportionately larger (and more efficient) wings that small birds.
      • - Heavier birds also have higher wing-loading than smaller birds, due to constraints associated with the anatomy and physiology of maintaining large wings don't keep up with incr. Mass.

    • 2) Habitats - wings can be classified into 4 types, each adapted for a different type of habitat and lifestyle.
      • (a) Elliptical = adapted for forest or shrub habitat where
        maneuvering is important. Low aspect ratio.
        (Galliformes, many Passerines).
      • (b) High Speed = adapted for feeding on wing or very long-
        distance migration; flattish profile, fairly high aspect
        ratio. [Shorebirds, Falcons, Hummers, Swallows, &
      • (c) High Aspect Ratio = long and narrow, adaptive for soaring
        (Pelagic birds.)
      • (d) High Lift = adapted for carrying heavy loads. Moderate
        aspect ratio, deep curvature (Hawks & Eagles, Cranes).

      Wing type and lifestyle are closely associated.

    - Birds derived from reptiles, modern birds share many primitive characteristics with their reptilian ancestors. (primitive = present in ancestors; derived = new evolutionary development, not present in ancestors)
    • 1) single occipital condyle (mammals with 2)
    • 2) jaw articulation (different bones in mammals)
    • 3) single middle ear bone (mammals have 3)
    • 4) scales on legs
    • 5) nucleated RBC's (mammals anucleate)
    • 6) egg-laying habits

    - Study of avian evolution must begin with the first known bird, Archaeopteryx, from Jurassic deposits (about 150 mya) in Germany.
    - Archaeopteryx was about the size of a crow, and it had:
    • 1) fully developed flight feathers exactly like modern birds
    • 2) a furcula (wishbone), which is an avian adaptation for powered flight
    • 3) a pelvis and legs similar to modern birds

    - However, most skeletal features were reptilian, and without feather impressions the fossils would have been classified as reptiles (more specifically, as a type of dinosaur known as coelurosaurs).
    - Shared primitive features of Archaeopteryx with reptiles:
    • 1) reptilian skull with teeth
    • 2) long, bony tail
    • 3) handbones (metatarsals and digits) not fused, digits also bore claws
    • 4) sternum unkeeled, probably cartilaginous
    • 5) ribs not fused with uncinate processes as in modern birds
    • 6) abdominal ribs present
    - Since Archaeopteryx lacked a keeled sternum, some have questioned whether it could fly. Answer is probably yes.

    • (1) Feather vanes are asymmetrical (SEE HANDOUT)
    • (2) Heavy furcula to which pectoralis probably attached
    • (3) Acute angle of scapula (as in modern flying birds, different from obtuse angle of flightless birds) - (SEE HANDOUT)

    - Consensus = capable of powered flight, but not powered takeoff. Recently, it has been suggested that Archaeopteryx may have had reptilian physiology, that would allow powered takeoff. This suggestion is based on the differing physiology between reptile and bird muscle and the differing morphology of the pectoral girdle and muscles used for takeoff in Archaeopteryx and modern birds. In modern birds, the supracoracoideus powers the upstroke but it originates on the keeled sternum and inserts on the dorsal humerus via a tendon passing through the triosseal canal (SEE HANDOUT).

    - Archaeopteryx lacks a triosseal canal and the supracoracoideus apparently inserts on the ventral humerus so it pulls the wing downward. The upstroke is presumably then powered by the deltoids, and a prominent acromion process may serve as the attachment for these muscle to the humerus.

    - In order to generate the power needed for takeoff, both pectoralis and deltoids may have had reptilian muscle physiology (high power, but rapid fatigue), that may have allowed powered takeoff, whereas avian muscle (lower power, but slow fatiguing) was probably not capable of generating the necessary power for takeoff.

    - Archaeopteryx serves as a great example of a transitional form between reptiles and modern birds. But, what does it tell us about the ancestry of birds?


    - 2 Hypotheses that are still vigorously debated today.

    • I. Pseudosuchian Thecodont Ancestry - Pseudosuchians were small, rather lightly built, bipedal reptiles present in the early Triassic about 200-230 mya. Gave rise to pterosaurs, dinosaurs and crocodilians (SEE HANDOUT).
      • - This hypothesis argues that birds arose from Pseudosuchians directly at least 200 mya. Pseudosuchians share many features with primitive birds: skull, elements of fore- and hindlimbs, elements of pelvic and pectoral girdles, ribs, tail vertebrae, ear sinuses, and teeth.

      • - Problems:
        • (1) Appears that there may be to wide of a gap between Pseudosuchians (200-230 mya) and Archaeopteryx (150-160 mya).
        • (2) Pseudosuchians are very primitive and generalized reptiles with very few shared derived characteristics with birds (ear sinuses and teeth have been claimed). Similarities are just retained primitive characteristics.

    • II. Coelurosaurian Theropod (Dinosaur) Ancestry - Coelurosaurs were lightly built, bipedal dinosaurs with elongated limbs adapted for running. They were contemporaneous with Archaeopteryx in upper Jurassic and lasted through Cretaceous (150-65 mya). (SEE HANDOUT)
      • - Coelurosaurs shared many derived (specialized) skeletal features with Archaeopteryx including: structure of vertebral column, forelimb and hand structure, hind limb structure, pectoral arch, and pelvis.
      • -Archaeopteryx would be classified as a coelurosaur without feathers showing in fossils.
      • - Problems: Coelurosaurs occurred at same time or later in fossil record than Archaeopteryx, so could not be direct ancestor. However, proponents argue that they may have shared a common ancestor.

    - 2 Hypotheses, again still vigorously debated.
    • I. Arboreal Hypothesis = "from the trees down"
      • - Argues that primitive birds (Archaeopteryx) were arboreal and developed the ability to glide and from gliding flight, flapping flight evolved secondarily.
      • - Evidence:
        • (1) Forelimb claws and foot structure presumably advantageous for climbing and perching.
        • (2) Long, flat tail seems more useful in arboreal situation.
        • (3) Easier to imagine, less energetically expensive.
      • - Problems:
        • (1) Extant gliders all use very different structures for gliding than the wings birds use for flight; gliding is a perfectly fine adaptation in its own right - difficult to imagine a selective factor causing the change from gliding to flying.
        • (2) Hind limbs of Archaeopteryx appear adapted for running.
        • (3) No specialized arboreal adaptations are apparent (although feet appear adapted for perching).

    • II. Cursorial Hypothesis = "from the ground up"
      • - Argues that bird flight evolved from running and leaping to catch insects. Flight feathers thought to have evolved to maintain stability while leaping. Even a small amount of lift would greatly increase stability. Flapping flight evolves as a continuation of this trend.
      • - Evidence:
        • (1) Hind limbs adapted for running.
        • (2) Wing morphology very different from gliding structures in other animals.
      • - Problems:
        • (1) Foot structure advantageous for perching not standing on ground (but a similar arrangement is also present in carnivorous theropod dinosaurs which were runners).
        • (2) Flight apparatus of Archaeopteryx maybe not well enough developed to permit takeoff from ground (although recall the reptilian muscle physiology suggestion).