Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180


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.


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


    • 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.


    • 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.