As in all vertebrates the urinary and reproductive systems are closely linked in development and in function. Birds retain many features in common with their reptilian cousins, albeit adapted to active homoeothermy:

(1) They secrete uric acid rather than urea. This allows for water conservation, but also permits storage of embryonic waste in the egg in solid rather than liquid (and toxic) form.

(2) A rudimentary renal portal system is retained, although this can be shut down during vigorous exercise when rapid venous return is required.

(3) The reproductive system shows many reptilian features including reductions in size and mass (for flight), strong seasonality and, of course, oviparity. Unlike many rodent eggs, however, the avian egg has a large yolk mass to support homoeothermic development, and a porous shell for gas and water exchange

Kidneys and ureters

The metanephric kidneys excrete dilute urate that then gets concentrated in the cloaca. The kidneys themselves are 3-lobed structures lying in the dorsolateral abdomen. The ureters arise from the midle lobe and pass to the cloaca. This itself has three zones from cranial to caudal:

* Coprodeum receiving the faeces

* Urodeum receiving the ureters and gonadal ducts.

* Proctodeum -- a common discharge area.

In some male birds the cloaca may be eversible to form a copulatory organ.

Salt excretion

In addition to kidneys that are highly efficient at eliminating nitrogenous waste and conserving water, marine birds have the problem of eliminating salt.

Birds along with at least three groups of diapsid reptiles (lizards, snakes and crocodiles) have evolved a system of lateral nasal salt glands. These are structured in a way reminiscent of the kidney, using a countercurrent capillary exchange mechanism surounding the secretory tubules of the gland. The single layer of cells surrounding each tubule actively secretes a fluid about 5 times more concentrated in salt than the body fluids.

In birds these nasal or supraorbital glands lie over the orbit and secrete brine into the nasal passages. From here it either drips or is sneezed out.

Male reproductive system

The males have paired abdominal testes lying cranioventral to the first kidney lobe. There is enormous variation during the breeding season.

The vas deferens emerges medially and passes caudally to the cloaca where it has a common opening with the ureter in the Urodeum. The terminal vas deferens is swollen as a storage organ: the seminal vesicle. See and complete Fig 5-1

As in mammals, sperm formation is temperature sensitive, and maturation is assisted by nocturnal drops in temperature, or by the development of scrotal-like external thermoregulatory swellings holding the seminal vesicles.

In addition, male birds tend to have relatively low extragonadal sperm reserves and sperm are ejaculated soon after production in the testes. In this way, it is thought, deleterious aging effects do not have time to accumulate.

Copulation in birds involves apposition of the cloacal region of male and female following courtship. Courtship itself may be intense and competition between males for access to females has been major driving force behind the development of elaborate plumage and song.

Many birds, however, are monogamous and show extreme degrees of cooperation in caring for young. Probably the most dedicated is the male Emperor Penguin, who incubates a single egg through the dark of an Antarctic winter.

Fig 5-1 Male Reproductive system (after Proctor & Lynch, 1993)

Female reproductive system

In most birds only the left ovary and oviduct persist. This reduces mass and makes for more efficiency as only one developing egg needs to be supported at any one time.

Atrophy of the ovary in old age, or removal by intervention or disease results in the development of the vestigial right gonad as an ovotestis. This secretes testosterone that causes masculinization of the bird.

As in the male, the ovary enlarges greatly during the breeding season. Active ovaies resemble pale bunches of tiny grapes -- the developing follicles. The oviduct opens medially to it in a funnel-shaped ostium - the equivalent of the mammalian infundibulum and fimbriae.

The oviduct receives the developing eggs and has distinct regions in which differing stages of egg maturation occur. Sequentially, these are the magnum, the isthmus, the uterus with pigment glands and the vagina for temporary egg storage.

Details of egg layer formation are given below. See and complete Fig 5-2.

Egg formation

Typically egg formation involves the following steps.

Ovulation releases an egg from a mature follicle on the surface of the ovary. The egg has extensive food reserves in the form of concentric layers of yolk. This is immediately picked up by the ostium, where ciliary currents carry it into the magnum region. Fertilization normally occurs here if the female has mated: sperm are stored for extensive periods in glandular regions of the lower oviduct.

Over about three hours the egg receives a coating of albumen.

The egg and albumen then pass into the isthmus, where the shell membranes are deposited. This takes about one hour.

In the uterine region the egg spends about a day, and pigment gets laid down in the shell in characteristic patterns. Final hardening of the shell occcurs here and the egg then passes into the vagina and cloaca for laying.

Fig 5-2 Female Reproductive System (after Proctor and Lynch, 1993)

Endocrine glands

The avian endocrine glands are responsible for coordinating the bird's response to stress, reproduction, metabolic demands and body growth. The basic pattern is very similar to mammals. See and complete Table 5-1.
    Gland       Location           Hormones               Functions                
Thymus and                                                                         
Bursa of                                                                           


In which birds does an erectile penis develop?

Why haven't birds developed true viviparity?

Many birds' eggs have characteristic pigmentation patterms. How do these get laid down and what advantage does the colouration give?

What advantage is there to sperm storage in the female?

If birds can get rid of excess salt by means of salt glands, how do marine mammals manage?

Page constructed by Jim Cummins and updated on April 1 1996.
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