Biology 363/563 Ornithology

Dr. David Swanson, Office: CL 180



** UROGENITAL SYSTEM **

I. EXCRETORY SYSTEM - functions in osmoregulation and nitrogenous waste removal. Consists of paired kidneys, drained by ureters into urodeum region of cloaca. No urinary bladder is present.

  • 1) Avian Kidney = consists of an outer cortex and an inner medulla, and is relatively larger than the kidneys of mammals and reptiles.

  • - Nephrons are of 2 types:

    • a) Reptilian Type = relegated to cortex; no concentrating loop present

    • b) Mammalian Type = tubules extend into medulla as loop of Henle; allows some concentration of salts (to levels slightly greater than those in the body fluids), but not nearly as effective as most mammals at concentrating urine.

  • - Blood supply to kidney (similar to reptile condition):

    • a) Arterial supply --> Glomeruli
    • b) Renal Portal Vein --> surrounds tubules
  • - Birds can also remove water from cloaca by first passing it to intestine where it is reabsorbed.

  • - Approximately 98% of the water filtered by the kidney is reabsorbed.

  • 2) Nitrogenous Waste Removal = nitrogen-containing wastes are secreted principally in the form of uric acid (a semisolid insoluble paste). Because it is insoluble in water, little water is required for its excretion.This is an important mechanism for water conservation in birds.

  • - Example:
    • Mammals --> 60 ml water to excrete 1 g urea (Kangaroo Rat, up to 30X blood levels)
    • Birds --> 2-3 ml water to excrete 1 g uric acid (3000X blood levels)

  • - Each molecule of uric acid contains 4 nitrogens, urea only contains 2. Tradeoff = uric acid also contains more carbon atoms than urea (5 vs. 1) so it is more energetically expensive to produce.

  • - Birds tend to lose more water via evaporation than mammals due to their higher metabolic rates.Nevertheless, a few desert-adapted birds can subsist solely on metabolically produced water + preformed water in their food, as do some reptiles and mammals.

  • 3) Osmoregulation - Although avian kidneys are good at producing concentrated nitrogenous wastes, they can't concentrates salts or electrolytes much above plasma levels (4-5X at best, mammals up to about 30X).

  • - Salt Glands = located near orbits, secrete a concentrated salt solution composed mainly of sodium and chloride ions, and possibly potassium ions. Solutions can be up to 5% salt. Salt glands have a widespread occurrence among nonpasserines with potential problems of salt imbalance from salty diets (e.g., pelagic birds).

  • - Mechanism of concentration involves active (ATP-requiring) transport of ions by Na+/K+ ATPase.Seems to involve removal of water from tubule (osmotically following salts) rather than direct addition of salts. Fluid entering peripheral tubules of salt gland is isosmotic (same concentration) with plasma due to countercurrent exchange and diffusion.



II. REPRODUCTIVE SYSTEM

    - Gonads + Ducts. Gonads are paired testes in males, single ovary in most females; produce gametes and secrete sex hormones.

    A. MALE SYSTEM

    • 1) Testes - enlarge greatly during reproductive season. May be 200-300X larger than at other times of the year.

    • 2) Vas Deferens - drain testes, also enlarge greatly during reproductive season.

    • 3) Urodeum - part of cloaca into which vas deferens empty

    • 4) Penis - present in Ratites, Tinamous, Storks, and Anseriformes as an erectile grooved structure fixed to the ventral wall of the cloaca. Functions to introduce sperm in the female reproductive tract.

    • - Most birds copulate via a "cloacal kiss" = brief cloacal contact between inverted cloacas.



    B. FEMALE SYSTEM

    • 1) Ovary - only the left ovary develops in most birds, but functional right ovaries occur in Accipitridae, Falconidae, Cathartidae, and Brown Kiwis.

    • 2) Oviduct - specialized to function in the development and production of eggs, increases in size by 10-50X during the breeding season.

      • a) Magnum = adds albumen (egg white protein)
      • b) Isthmus = adds shell membranes, keratin fibers present
      • c) Uterus (Shell Gland) = adds external shell, mainly composed of CaCO3 (also Mg2+, PO42- as minor components).
      • d) Vagina = powerful muscles for egg expulsion

    • 3) Urodeum region of cloaca serves as exit of reproductive system to exterior.



    • - Copulation introduces sperm into lower oviduct; sperm swim directly to upper end where fertilization occurs, before membranes and shells are added.


    ** NERVOUS SYSTEM **

    Central Nervous System (CNS) = brain and spinal cord (responsible for integration and memory).

    Peripheral Nervous System (PNS) = cranial and spinal nerves, autonomic nervous system, sense organs (both sensory and motor components).

    I. CENTRAL NERVOUS SYSTEM

    1) Brain = same basic plan as in reptiles and mammals, 3 Divisions:

    • a) Forebrain (Cerebrum) = integration, instinctive behavior, intelligence
    • b) Midbrain = vision, muscular coordination, physiological control
    • c) Hindbrain (medulla) = links brain with spinal cord and peripheral nervous system


    - Birds and mammals both have enlarged cerebral and cerebellar hemispheres; the brain in both Classes makes up 2-9% of total body weight.

    A) Forebrain

    - Different regions of cerebrum become enlarged in birds and mammals. In mammals, the outer layer (cortex) greatly enlarges to become dominant and serves as the seat of higher intelligence. Provides a great capacity for learning.

    - In birds, the cortex is thin and relatively undeveloped (thought to be the seat of conditioned behavior); The corpus striatum underneath grows to become dominant (serves as the seat of learning and intelligence as well as complex instinctual behaviors. SEE PG. 181, GILL.

    - In general, the cortex appears specialized for learning, the corpus striatum for stereotypic behaviors; Recent evidence, however, suggests that birds are more capable of learning than previously thought and can outperform mammals in some advanced learning experiments (e.g., counting).

    B) Midbrain

    - Birds with large, well-developed cerebellum (largest among the vertebrates), associated with very high degree of muscular coordination necessary for flight.

    - Very large optic lobes are present, associated with the importance of vision in birds.

    C) Spinal Cord - similar in structure to other tetrapods, cervical and lumbar enlargements associated with appendages.


    II. PERIPHERAL NERVOUS SYSTEM - similar to the situation in other vertebrates.


    III. SENSES

    1) Smell - olfactory lobes are generally small and birds have a generally poor sense of smell. Some birds, however, are capable of smelling very effectively (e.g., Turkey Vultures, kiwis).

    2) Taste - all birds can taste; birds are equally or less sensitive to certain ingredients than mammals.Birds have fewer taste buds than do mammals.

    3) Mechanoreception
      a) Touch - possess typical touch, pressure, temperature, and pain receptors.
    • b) Birds are also sensitive to barometric pressure.
      • (i) Many birds can sense oncoming storms and modify foraging behavior accordingly.
      • (ii) Pigeons and thrushes can select proper altitude for migratory flights, presumably this is true for other birds as well.
    • c) Magnetism - birds can use information from the earth's magnetic field for navigation. Magnetite Crystals are present near olfactory nerves (between eyes) of pigeon, and these may serve as the basis for the magnetism-detection system.


    IV. HEARING

    • a) Ear is divided into same 3 regions as in mammals: External, Middle, and Internal.
    • - Middle Ear is only one bone (columella) = transmits sound vibrations from tympanum to inner ear.
    • - Inner Ear serves both hearing and equilibrium functions.
    • b) Optimal Hearing Range = 1 - 5 KHz, Limit = 10 KHz; Owls to lower frequencies and up to 12 KHz. Overall, the range of optimal hearing in birds is narrower than that in mammals. (SEE PG. 194, GILL).

    • c) Owls with specializations allowing them to detect and capture prey by hearing alone.
      • (i) Detect low frequency sounds effectively
      • (ii) Have high numbers of auditory neurons
      • (iii) Facial discs act as sound collectors and aid in focusing sound to ear
      • (iv) Asymmetry of external ears - allows binaural comparison of intensity and frequency, which enables precise vertical distinction in addition to horizontal distinction similar to ours. Because of this they can capture prey by sound alone.

    • d) A few birds are capable of echolocation for navigation (Cave Swiftlet, Oilbird). Use low frequency clicks. This differs from the high frequency ultrasound used by bats and is not nearly as effective.


    V. VISION

    1) Vision is the most important sensory input for birds, as they are visual animals.

    2) Birds have large eyes relative to other vertebrates (e.g., starlings have an eye that makes up 15% of the head mass, humans = 1%)

    3) Shapes of avian eyes vary.

    • a) Globular = diurnal birds with high resolution over great distances (hawks, etc.)
    • b) Flattened = most birds
    • c) Tubular = nocturnal birds, allows increased accommodation (focusing) and light-gathering

    4) Birds with higher visual acuity (resolving power) than mammals because of higher numbers of photoreceptors and a slight magnifying effect of the fovea.

    - Raptors and passerines = 2 - 3 times human abilities


    5) Generally, birds have higher powers of accommodation as both the cornea and the lens change curvature while focusing. Only the lens changes curvature in mammals.

    6) Color Vision - birds have very high numbers of cones (diurnal birds), which suggests well-developed color vision.

    - Birds are sensitive to UV light

    - Diurnal birds have colored oil droplets in the eyes, probably functions to enhance contrast by filtering out background "noise"

    7) Birds do not have stereoscopic vision as do mammals. Optic nerve tracts project only to the opposite brain hemisphere.

    - Most birds see laterally better than forward due to the lateral position of eyes on the head and little overlap in fields of view.

    8) Pecten = structure composed of blood vessels and supporting stromal cells, present at exit of optic nerve from eye and projects into vitreous chamber of eye

    - May serve a nutritive role for the avascular retina since it is highly vascularized, but the exact function is unknown. Other proposed functions include: (1) reduce glare, (2) regulate pressure or temperature within the eye, (3) perception of movement, (4) light absorption.