I. General Characteristics.

    A. 2 wings and 2 legs.
    B. Covered with feathers (the only animals with feathers).
    C. Strong, light-weight skeleton.
    D. Rapid, efficient digestion.
    E. High metabolism.
    F. Endothermic.
    G. Air sacs.
    H. 4-chambered heart.
    I. Oviparous.
    J. Internal Fertilization.

II. Feathers.

    A. General Characteristics.
        1. Grow from the papilla in the skin.
        2. A large bird may have up to 25,000.

    B. Parts.
        1. Shaft.
            a. Quill.
            b. Rachis.
        2. Vane.
            a. Parallel rows of barbs perpendicular to the shaft.
            b. Parallel rows of barbules perpendicular to each barb.

    C. Care.
        1. Stroking reattaches the barbules.
        2. Preening is the application of oil.
            a. Prevents the feather from becoming brittle.
            b. Waterproofs the feather.
            c. Gland at the base of the tail produces oil.

    D. Types.
        1. Down.
            a. Barbules do not interlock.
            b. This makes them fluffy instead of smooth.
            c. Provide insulation.
        2. Contour.

    E. Molting.
        1. Often occurs in late summer before migration.
        2. Flight feathers molt 2 at a time, one on each side, to maintain balance.

III. Wings.

    A. Elliptical.
        1. Short and wide.
        2. Quick take-off and landing, low-speed flight, great maneuverability.
        3. Sparrow, woodpecker.

    B. High-speed.
        1. Thin, long, tapered.
        2. Minimum drag.
        3. Tern, swift.

    C. Soaring.
        1. Long, thin, like a glider.
        2. Stay aloft with minimum energy.
        3. Gull.

    D. High-lift.
        1. Large, convex, great lift.
        2. Enable to carry large prey.
        3. Hawk, owl, eagle.

IV. Feet.

    A. Wading.
        1. Long, thin, widely spaced toes; thin legs.
        2. Distribute weight to prevent sinking in the mud.
        3. Move through water without creating turbulence.
        4. Stork, crane, flamingo.

    B. Swimming.
        1. Webbed feet.
        2. Paddle in water.
        3. Duck, goose.

    C. Climbing.
        1. 2 front toes, 2 rear toes; short and sharp.
        2. Grip vertical surfaces.
        3. Woodpecker.

    D. Running.
        1. Long, thick, muscular.
        2. Ostrich.

    E. Grasping.
        1. Talon-tipped.
        2. Clamp onto and pierce prey.
        3. Falcon, eagle.

    F. Perching.
        1. 3 front toes, 1 rear toe, with a “locking device.”
        2. Keeps bird on its perch, even while sleeping.
        3. Robin, wren.

V. Beaks.

    A. Spearing.
        1. Long, slender.
        2. Spear fish in the water or pluck insects from tree bark.
        3. Stilt.

    B. Cracking.
        1. Short, thick.
        2. Cracking and crushing seeds.
        3. Cardinal, bunting.

    C. Predatory.
        1. Hooked, pointed.
        2. Tearing prey apart.
        3. Hawk, eagle.

    D. Filtering.
        1. Large, with a built-in sieve.
        2. Separate tiny plants and animals from water.
        3. Flamingo.

VI. Skeleton.

    A. Jaws are elongated to form the bill.

    B. Many neck vertebrae.
        1. Very flexible neck.
        2. Useful in manipulating objects with the head.
        3. Compensates for immovable eyes.

    C. Trunk vertebrae, ribs, and sternum (breast bone) are fused to make a rigid “cage.”

    D. Large sternum has a ridge (keel) where flight muscles are attached.

VII. Digestion.

    A. High metabolism required by high body temperature and high energy demands of flight.
        1. Young birds may eat more than their own weight in a day.
        2. Rapid digestion.
            a. Thrush digests berries in 30 minutes.
            b. Shrike digests a mouse in 3 hours.

    B. Digestive tract.
        1. Esophagus.
        2. Crop: storage.
        3. Stomach: secretes digestive juices.
        4. Gizzard: grinds food.
        5. Intestine: absorbs nutrients.
        6. Rectum.
        7. Cloaca.

VIII. Respiration.

    A. Air sacs attached to lung.
        1. 25% of incoming air goes to the lung, 75% goes to the air sacs.
        2. Air sacs empty air into the lungs during exhalation.
        3. Thus O2 is absorbed during both inhalation and exhalation.
        4. Air sacs also lighten the weight of the bird.

    B. Respiration is bird’s primary means of cooling the body.

    C. Songs are produced by the syrinx, an enlargement of the trachea.

IX. Circulation.

    A. 4-chambered heart keeps oxygenated blood separated from deoxygenated blood.
    B. High metabolism requires rapid circulation: 135 to 1,000 beats per minute.

X. Senses.

    A. Good hearing.
    B. Best eyes in the Animal Kingdom.
        1. Usually immovable.
        2. Usually set on either side of the head.
            a. Wide field of vision.
            b. Poor perspective.
            c. Some, like the owl, have eyes in the front of the head and do have perspective vision.

XI. Reproduction.

    A. Courtship.
        1. Dazzling appearance.
        2. Song.

    B. Internal fertilization of amniotic eggs.

    C. Care of the young.
        1. Most eggs require incubation, usually by parent(s) sitting on them.
        2. Altricial chicks.
            a. Less than 2 weeks incubation.
            b. Hatch naked, blind, helpless.
            c. Parents feed until they are feathered and able to fly.
            d. Less than 6 eggs per year.
            e. Robin, thrush, sparrow.
        3. Precocial.
            a. Longer incubation.
            b. Hatch able to move and feed themselves.
            c. Stay close to parents for protection.

    D. Reproductive Cycle.
        1. Usually 1 year.
        2. Most birds choose a different mate each year.
        3. Some, like eagle, mate for life.

XII. Migration.