Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180


  • 1) 4-chambered Heart - necessary to pump blood rapidly through vessels to meet high metabolic demands.
    • a. Bird hearts are 50-100% larger than hearts of mammals of similar body size.
    • b. Resting heart rates of birds are generally lower than those of similar- sized mammals; birds have higher stroke volume (amount of blood pumped per heart beat) than similar-sized mammals, so total cardiac output (amount of blood pumped per time) is generally similar.
    • c. Blood pressure is generally somewhat higher in birds than in mammals.
    • d. Bird heart also differs from mammal heart in the left ventricle pumps blood to the body via the right systemic arch (left arch drops out during embryonic development; in mammals the left systemic arch persists and the right drops out.

  • 2) Vessels = similar pattern to mammals and reptiles (SEE HANDOUT).
    • a. Aortic Arches - 6 in early embryo; 1, 2, and 5 drop out.
      • #3 --> Carotid Arteries to head
      • #4 --> Only right 4th persists as systemic arch
      • #6 --> Pulmonary arteries
    • b. Large brachial and pectoral arteries which supply blood to flight musculature.
    • c. Renal portal Vein persists (present in jawed fish, amphibians, reptiles, but drops out in mammals). These veins drain the posterior part of the body trunk and enter the capillary bed surrounding the kidney tubules.
  • 3) Blood - consists of plasma and formed elements (cells)
    • a. Plasma = 80% water, 0.9% NaCl, glucose concentration is 2X higher than in mammals (adaptive for support of high metabolic rates).
    • b. RBCs = nucleated as in all vertebrates except mammals, Hematocrit (packed cell volume) averages about 40% but varies among species, sexes, seasons, altitude, etc.
    • - Hemoglobin concentration lower (slightly) than in mammals; oxygen affinity (how tightly oxygen is bound) is generally lower in birds than in similar-sized mammals, and this enhances unloading of oxygen to tissues.
    • c. WBCs = main function is defense against foreign pathogens, immune response; similar types and functions to mammals.
    • d. Thrombocytes = cells involved in clotting, functional analogues to mammalian platelets.
  • 4) Lymph System is poorly developed compared to mammals.
    • a. Valves are present in lymph vessels to prevent backflow. Lymph is moved by skeletal muscle contraction, as in mammals.
    • b. Lymph Hearts - present in all bird embryos, also in reptiles and amphibians (as adults); usually disappear in adults, but persist in a few species - Ostrich, Cassowaries, Gulls, Storks, some Passerines.


    • A. FUNCTION = gas exchange, oxygen to tissues, carbon dioxide from tissues to lungs.
      - To meet the high oxygen demand of flight, birds have the most efficient respiratory system of any vertebrate. It differs from the respiratory system of mammals in several aspects.
      • 1) Comprised of a system of cranial and caudal air sacs that are located anterior and posterior to the lungs, respectively. Air sacs are thin sacs continuous with the lungs but not involved in gas exchange. They are present within the body cavity and extend into many pneumatic bones. The connect directly to primary and secondary bronchi.
      - Most birds have 9 air sacs:
      • a) paired cervical sacs in neck
      • b) paired anterior thoracic sacs
      • c) paired posterior thoracic sacs
      • d) paired abdominal sacs (extend into pneumatic bones of legs and pelvis)
      • e) single interclavicular sac (penetrates wing bones, sternum, and syrinx)
      • 2) One-way air flow through the lungs.
      • 3) Lungs are spongy in structure, rather than sac-like.
      • 4) Voice produced in the syrinx = unique vocal organ of birds. (SEE HANDOUT)

      - Gas Exchange occurs in air capillaries that open to a tube (parabronchus) through which air passes in 1 direction. 2 complete inspiration/expiration cycles are required for a single bolus of air to move through the respiratory system. SEE HANDOUT

      - Blood flow in the avian lung is via a crosscurrent mechanism relative to air flow. This allows very efficient extraction of oxygen from air. SEE HANDOUT

      - Avian respiratory system allows normal activity at high altitudes. Some geese are known to migrate over the top of the Himalayas (30,000+ feet). Tucker (1968) exposed mice and house sparrows to a simulated altitude of 20,000 ft. Mice were comatose, sparrows were able to fly normally. Mice and sparrows had similar body mass, similar metabolic rates, and similar blood oxygen affinity, so the difference is most likely explained by the increased efficiency of oxygen extraction.

      - Breathing is accomplished by a negative pressure system (as in mammals); involves expansion of thoracic cavity to decrease pressure, air rushes in to equilibrate.

    • B. SYRINX = vocal organ in birds, located at bifurcation of trachea into bronchi. No syrinx is present in vultures, ostriches, Mute Swan, some storks.
      - 3 General Types:
      • 1) Tracheal = forms from tracheal tissues. Has only 1 pair of membranes on lateral walls which constrict the single air passage (chickens, ducks, parrots, antbirds).
      • 2) Bronchial = from bronchial tissues, similar structure to above, except paired because there are two bronchi (cuckoos, nightjars, some owls).
      • 3) Tracheobronchial = from both, occurs in most birds; Oscine passerines have 2 membranes extending from lateral walls of each bronchus (SEE PG. 224-225, GILL)
      • Syrinx Musculature = controls syrinx action during sound production; pressure from interclavicular air sac also is important in constricting tubes.
      • 1) Most nonpasserines have only 2 extrinsic muscles associated with the syrinx.
      • 2) Oscine passerines have up to 6 pairs of intrinsic muscles in addition to the extrinsic muscles. Each half can produce different complex songs at the same time.


    - Birds lack teeth so they can't chew their food, the digestive tract is specialized to digest unmasticated food.
    1. Bills (or Beaks) = used for acquisition and ingestion of food. Specialized for the type of food eaten. They may also function secondarily in display (e.g., puffins, toucans, pelicans, etc.). (SEE HANDOUT)

    2. Oral Cavity = ingestion; contains taste buds, pressure receptors, specialized tongue (depending on source of food utilized). SEE HANDOUT. Also has mucous glands (esp. in birds eating dry food for lubrication) and salivary glands (secretions for carbohydrate digestion, sticky substance used by many birds in nest building or insect trapping).

    3. Esophagus = main function is food passage to stomach, contains many mucous glands.
    - Specializations:
    • a) Crop = outpocketing of esophagus to store food (prominent in granivorous birds).
    • b) Pigeons produce nutritious fluid for young in esophagus ("pigeons milk").
    • c) Serves as a resonating chamber for sound production and display in several birds (e.g., ostrich, grouse, pigeons). SEE PG. 168, GILL

    4. Stomach = bipartite. SEE PG. 167, GILL

    Anterior = Proventriculus (glandular stomach) - secretes acid and gastric juices

    Posterior = Gizzard (muscular stomach) - responsible for grinding food; internal surface covered with keratin, small pebbles (grit) taken into the gizzard of many birds (esp. granivorous) to aid grinding = analogue of mammalian molars.

    - Gizzard not as muscular in species eating meat, insects, or fruit.

    - Some birds that show seasonal changes in diet show corresponding changes in gizzard structure.

    5. Intestine = functions in chemical digestion and absorption; tends to be much longer in birds eating plant material than in those eating animal matter; length may change seasonally in response to changing diets.

    - Cecae = sac-like extensions of posterior intestine. Prominent in ostrich, cranes, waterfowl, Galliformes. Virtually absent in arboreal birds. Function is uncertain, but appears to be involved in water and protein absorption and in bacterial breakdown of cellulose (hindgut fermentation). This latter function is probably correlated with high rates of intestinal reflux that have been measured in some birds.

    6. Liver & Pancreas = add digestive juices for chemical digestion, empty to anterior region of intestine.

    7. Cloaca = divided into 3 regions:
    • 1) Coprodeum = anterior region, receives excrement from digestive tract.
    • 2) Urodeum = middle region, receives waste from urinary system and gametes from reproductive system.
    • 3) Proctodeum = posterior region, stores digestive and excretory system waste; closed by a muscular sphincter posteriorly.

    Hummingbirds 97-99% of energy in nectar is assimilated
    Raptors66-88% of energy in meat is assimilated
    Granivores 49-89% of energy in seeds is assimilated, dept. on seed type
    Herbivores 60-70% of energy in young plants is assimilated
    30-40% of energy in mature foliage is assimilated

    - Hoatzin is obligate folivore (eats leaves) from South American tropics (Cuculiformes); utilizes foregut fermentation with bacteria (similar to ruminant digestion in some mammals). Digestive efficiency on mature leaves is about 71%. An enlarged muscular crop and lower esophagus serve as the main fermentative structures. SEE PG. 167, GILL