Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180


  • 1) Protection (esp. of vital organs)
  • 2) Support (posture, flight, etc.)
  • 3) Articulation of appendages
  • 4) Blood cell production (marrow)
  • 5) Calcium reservoir (for eggshell production, muscle contraction, etc.)

II. AXIAL SKELETON = associated with central axis of the body (SEE HANDOUT)
  • A) Skull
    • 1) Large orbits are present to accommodate large eyes necessary for good vision.
    • 2) Single occipital condyle articulating with vertebral column (2 in mammals and amphibians, only 1 in birds and reptiles).
    • 3) Highest degree of cranial kinesis of any vertebrate group. Cranial Kinesis = both jaws (upper and lower) are hinged at their base so they can move independently of the skull, this allows a very wide gape and enhances ability to manipulate objects with the bill, important because birds don't have hands.
  • B) Vertebral Column and Ribs
    • 1) Rigidly fused in all regions but neck to provide stiffness during flight.
    • 2) Heterocoelous (saddle-shaped) centrum in cervical vertebrae allows high degree of neck mobility; 11-25 cervical vertebrae are present (vs. 7 in mammals).
    • 3) Uncinate Processes = posteriorly directed processes from ribs that act to fuse rib cage together and serve as an attachment for scapular muscles.
    • 4) Synsacrum = lumbar, sacral, and a few caudal vertebrae are fused into a solid structure, united with pelvis; allows support of the body by the legs.
    • 5) Pygostyle = fused caudal vertebrae at end of reduced tail; provides site of attachment for tail feathers.
III. APPENDICULAR SKELETON = associated with appendages (SEE HANDOUT)
  • A) Pectoral Girdle
    • 1) Forms tripod of bones to transfer energy from wing thrust to body.
      • a) Scapula = runs parallel to spinal column, braces shoulder against displacement caused by flapping.
      • b) Coracoid = braces sternum against compression resulting from contraction of pectoralis, prevents collapse of rib cage.
      • c) Furcula = strut which braces wings apart.
  • B) Wing = modified forelimb
    • 1) Folds against body in Z-shape for compactness.
    • 2) Humerus (upper arm) is shortened and broadened.
    • 3) Reduced number of wrist, hand, and finger bones that are elongated and fused.
  • C) Pelvic Girdle
    • 1) Lengthened and fused to synsacrum, forms roof covering about half of the body. Effective in providing support of the body by the legs.
    • 2) Pubis and Ischium open ventrally to accommodate centrally placed abdomen, also facilitates egg-laying.
  • D) Legs
    • 1) Spaced far apart by broadening of pelvis.
    • 2) Femur (upper leg) buried within flesh of body and directed forward so that legs are positioned under the center of gravity.
    • 3) Cnemial Crest = tibial extension at knee joint, especially well developed in divers; allows oar-like leverage from leg muscle for swimming.
    • 4) Some tarsal bones are fused to the distal tibia = tibiotarsus.
    • 5) Elongated heel = tarsometatarsus (tarsus), unfeathered portion of leg in most birds.
    • 6) 2, 3, or 4 toes in modern birds.


  • 1) Movement and posture
  • 2) Thermogenesis (shivering, possibly also non-shivering thermogenesis [NST])
- In birds, the majority of the musculature is shifted ventrally for improved aerodynamics. The rigidly fused vertebral column allows this shift and the associated reduced dorsal muscle mass.


  • 1) Flight Musculature - forms the largest musculature component in flying birds
    • a) Pectoralis - powers downstroke of wing, originates on pectoral girdle and partly on keeled sternum and inserts on ventral humerus. By far the largest muscle in the body of flying birds. Generally makes up about 15-25% of total body mass.
    • b) Supracoracoideus - powers upstroke of wing, originates on sternal keel and inserts (after passing through triosseal canal) onto dorsal humerus. Much smaller than pectoralis, but can be up to 11.5% of total body mass in hummingbirds (Due to hovering flight, hummers have a supracoracoideus about 5 times larger than most other birds, relative to body mass).
  • 2) Birds which rely more on running than flying and foot-propelled swimmers and divers have relatively large leg muscle masses.

    --> TAKE HOME: The size and distribution of muscle in birds is related to their lifestyle. (SEE HANDOUT)

  • 1) Slow, Tonic Fibers = slow contracting, fatigue-resistant, low energy usage and therefore low power; highly aerobic with high numbers of mitochondria (produce ATP using oxygen). Also known as Red Muscle (examples = postural muscles).
  • 2) Fast-twitch Glycolytic (FG) Fibers = fast contracting, fatigue rapidly, few mitochondria, high activities of glycolytic enzymes. Also known as White Muscle.
  • 3) Fast-twitch Oxidative-Glycolytic (FOG) Fibers = fast contracting, relatively high activities of both oxidative and glycolytic enzymes, high mitochondrial numbers, fatigue-resistant. Also known as Intermediate Muscle.
  • - Different muscles show varying ratios of the different fiber types (e.g., chicken breast is mainly FG, leg muscles show higher concentrations of FOG and ST fibers.
  • - Different birds also show varying ratios of the different fiber types in the same muscle (e.g., pectoralis in many gallinaceous birds is largely FG, in passerines it is almost entirely FOG).
    --> Again, muscle fiber type ratios in different muscles (esp. flight muscles), like the distribution of muscle within the body, reflect the lifestyle of the bird.

  • 1) Shivering = principle (if not exclusive) mechanism for heat production in birds. Flight muscles are primary sites for shivering thermogenesis, leg muscles may also be important.
    • a) Birds shiver isometrically = antagonistic muscle groups work simultaneously so very little surface disturbance occurs and, therefore, very little disruption of insulation at the surface.
    • b) Both burst shivering and continuous shivering have been reported in different birds.
    • c) For burst-shiverers (e.g., Japanese Quail), as temperature decreases both the amplitude and duration of shivering bouts increases. Cold-acclimation results in decreasing amplitude for the same level of cold stress. This is adaptive in that: (1) less disruption of insulation may occur, and/or (2) fatigue-resistant fibers are preferentially recruited.
    • d) Aerobic muscle appears to be recruited for shivering before anaerobic muscle in chickens. Whether or not this is typical for birds in general is not known.
  • 2) Non-shivering Thermogenesis (NST) = heat production without shivering.
    • a) Common in mammals (brown fat, futile cycles, or loosely coupled mitochondria in muscle). (SEE HANDOUT)
    • b) Has been reported in some birds, based on a discrepancy between oxygen consumption and electrical recording of muscle activity (EMG), but its occurrence is disputed. The mechanism and location of NST, if present, are unknown.
    • It is known that birds lack brown fat.
    • If present, likely tissue for NST is skeletal muscle.
    • Avian uncoupling proteins occur in skeletal muscle, but  they are not the same protein as the uncoupling protein in Brown Fat. Avian uncoupling protein function seems related more to oxidative stress than to heat production.