BIO 554/754

Avian Reproduction: Anatomy & the Bird Egg 

An updated version of these notes can be accessed from a new "Avian Biology' page


Reproductive Anatomy:

From: Akins and Burns (2001)
Sperm production

Light photomicrograph of a section of a testis showing a seminiferous tubule during full
semen production. SG indicates spermatogonia; PS, primary spermatocyte; Ss, secondary spermatocyte;
MS, mature spermatocyte; and L, lumen (original magnification ×800) (Samour 2002).

Male birds have paired abdominal testes lying cranioventral to the first kidney lobe. Testes increase dramatically in size 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 glomus (or seminal vesicle as in the drawing to the right). 

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 glomera. 

In addition, male birds tend to have relatively low extragonadal sperm reserves and sperm are ejaculated soon after production in the testes.



Cloacal protuberance

Longitudinal section of the cloaca of a male budgerigar during the culmination phase of the breeding cycle. SG indicates seminal glomus; P, proctodeum; and C, cloaca (original magnification ×12) (Samour 2002).

Testis size increases with colony size in Cliff Swallows -- By using a sample of over 800 male Cliff Swallows (Petrochelidon pyrrhonota) that died during a rare climatic event in their Nebraska study area in 1996, Brown and Brown (2003) investigated how testis size was related to body size, age, parasite load, and a bird's past colony-size history. Testis volume increased with body size. After correcting for body size, testis volume was lowest for birds age 1 and 2 years but did not vary with age for males 3 years old or more. Birds occupying parasite-free (fumigated) colonies had significantly larger testes than did birds at nonfumigated sites. Testis volume increased significantly with the size of the breeding colonies a bird had used in the past. These results show within a species that larger testes are favored in more social environments, probably reflecting a response to increased rates of extrapair copulation (and thus sperm competition) among Cliff Swallows in large colonies. The presence of ectoparasites, by inflating levels of plasma corticosterone, may in turn reduce testis mass. These data provide no support for the hypothesis that large testes, perhaps by producing more testosterone, are immunosuppressive and thus costly for that reason.

Egg production

Avian Ovary

Ovary,oviduct, & egg with shell

   In most birds, only the left ovary and oviduct persist. The ovary enlarges greatly during the breeding season. Active ovaries resemble bunches of tiny grapes -- the developing follicles. The oviduct opens medially to it in a funnel-shaped ostium. Ovulation results in the release of an egg from a mature follicle on the surface of the ovary. The egg, with extensive food reserves in the form of concentric layers of yolk, is picked up by the ostium and ciliary currents carry it into the magnum region. Over about three hours the egg receives a coating of albumen.
   The egg then passes into the isthmus, where the shell membranes are deposited. This takes about one hour. The egg them moves to the uterus, or shell gland, where the calcareous shell is added and, in some birds, pigment is added in characteristic patterns. The egg then passes into the vagina and cloaca for laying.

The left egg found inside the female oviraptorosaurian. The texture of the shell pieces probably resembles the original texture of the egg. Credit: Yen-nien Cheng
Eggs discovered inside dinosaur -- The discovery of eggs inside a dinosaur has provided new clues about dinosaur reproductive biology and more support for the hypothesis that birds evolved from dinosaurs. The pair of eggs are the first found inside a dinosaur. Sato et al. (2005) found that the dinosaur produced eggs in some ways like a crocodile and in other ways like a bird. Crocodiles have two ovaries enabling them to lay a clutch of eggs. Birds have a single ovary and lay only one egg at a time. The dinosaur's egg-producing capability lay somewhere in between, suggesting a possible link with modern birds. It had two ovaries, but produce only one egg at a time from each ovary.
    Sato et al. (2005) studied a dinosaur from a group of dinosaurs called oviraptorosaurians. This type of dinosaur — probably 3 - 4 meters long — is a subgroup of the theropods. The dinosaur was excavated in China. The similar size of the eggs suggests the creature's two oviducts each produced a single egg at the same time.

Female birds can bias the sex of their chicks.-- Whether a bird is more likely to lay a male or female egg depends on which sex will have the greatest chance of doing well. Rutstein et al. (2004) adjusted the food intake of female Zebra Finches [see photo of female (left) and male (right) Zebra Finches below right] & found that well-fed females were more likely to produce daughters, while less well nourished birds were more likely to have sons. This is exactly as predicted by the fact that female offspring need to be better nourished than males if they are to survive and grow well. 
      The authors noted that: “In most animals sex ratio is close to 50:50 and extremely resistant to change. In mammals, including humans, the sex of the baby is determined by whether the sex chromosome in the sperm is male or female. But in birds, it is the female’s egg rather than the male’s sperm that determines what sex the chick will be. Thus the female has the potential to determine the sex of her young by whether she ovulates male or female eggs. In some way, female Zebra Finches seem to be able to exert control over whether to produce a male or female egg depending on which of the two is most likely to be successful. Our research tells us that they do it, and we understand why. The big question is: how do they do it?” 
      In many animals, females need to be well-nourished and in good condition if they are to breed, as eggs are costly to produce. Bigger eggs tend to lead to bigger young that are more likely to survive. Such ‘sex ratio adjustment’ is well documented in certain insects, such as bees and wasps, but is less well understood in birds and mammals. 
      Birds are an excellent model to use in the study of sex ratio adjustment because, using molecular techniques, scientists can establish the sex of each egg soon after laying. Further, all the resources given to the developing embryo are present in the egg at laying. Thus the size and the content of the egg are measures of the amount of resources that the female has allocated to that egg, which affects its subsequent survival chances. 
     The authors explained: “We manipulated the diet quality of Zebra Finches to look at the effects of body condition on female investment. We found that females were able to exert a strong degree of control over the production of male and female eggs. When females were fed on a low quality diet, they laid eggs that were considerably lighter than those laid when they were fed on a high quality diet, and they also laid far more male eggs on a low quality diet. This is the converse situation to that described 30 years ago for mammals, but it makes sense for Zebra Finches. Previous research has shown that under poor nutritional conditions, female Zebra Finches grow more slowly and survive less well compared to males. Therefore, females are producing more of the sex with the highest survival chances under those conditions.” 

Two potential mechanisms for 
determination among birds. (A) the 
presence of the W chromosome 
triggers femaleness or (B) the 
presence of two Z chromosomes 
confers maleness.
Avian sex determination (Ellegren 2001) -- The molecular determinants behind sexual development in birds remain a mystery. The process is known to be different from that in mammals, with no homolog to the gene that confers maleness in mammals found in birds. The failure to identify such a gene in birds is probably a reflection of the fact that, despite the occurrence of two sexes being nearly universal throughout the animal kingdom, the genes involved seem virtually unrelated among metazoan phyla. These differences raise obstacles for comparative or candidate gene approaches in studies of sexual development. 
      In birds, females are the heterogametic sex, with one copy each of the Z and W sex chromosomes. Males are homogametic (ZZ). However, it is not clear whether it is the presence of the female-specific W chromosome that triggers female development, or the dose of Z chromosome that confers maleness.  An intriguing additional possibility is that both Z and W matter! In marsupials, for example, Y acts as a dominant testis determining chromosome, while the X chromosome determines the choice between pouch and scrotum. Maybe a system where the two sex chromosomes mediate different aspects of sex differentiation is also used in birds.

Copulation & fertilization:

Phony phallus puts sperm ahead in bird first--  "These birds would be at it for 10-20 minutes," said co-author Tim Birkhead of the Red-billed Buffalo Weaver. Males use their organ to rub females and improve their sperm's chance of success. Few male birds have a phallus; most achieve fertilization via a cloacal kiss. So 19th-century reports of a mock member in the Buffalo Weaver sent Winterbotton et al. (2001) to Namibia. Catching the birds in the act was tough, recounts Birkhead: "In 3 years we saw eight matings." Pairs occasionally emerged from nests and flew to a nearby tree. "I'd run after them, sweating profusely with my binoculars steaming up," he says. The pair would start bouncing up and down - over numerous consecutive bouts. Compared to the 1-2 second tryst of most birds, their staying power is unique. Yet, entry of the elusive organ was hard to make out. Even in captivity "they performed beautifully," but the view was blocked, says Birkhead. So they glued a piece of cardboard to an unlucky bird's member. This did not prevent mating, suggesting that the Buffalo Weaver organ is actually a weapon in sperm wars. By choosing a male who rubs longest or best, females may be selecting top-quality sperm. Paternity testing revealed that female Buffalo Weavers sire birds from multiple males, providing evidence of sperm competition. Time spent courting must be shown to predict sperm transfer or success to really back up the idea. The 1.5-cm appendage lacks blood vessels and has a twisted furrow down its length. Males in communal nests have longer ones than those that live alone, showing that size is a factor in social success. But for males at least, the phallus is for more than foreplay.  -- Helen Pearson, Nature Science Update

Additional photos:


Phalloid organ of a male buffalo weaver
(Photo source:

Diagram of the left lateral view of a retracted and erect phallus of a male emu or rhea. The top drawing
     represents the phallus within the pouch. A. vas deferens, B. urideum, C. proctodeum, D. pocket to contain phallus,
E. erectile wall of phallus, F inverted hollow tube of phallus, G. phallic sulcus, H. erectile tissue, and I. erect phallus
with blind hollow tube.  (Source:

Near the junction of the vagina and shell gland of female birds are deep glands lined with simple columnar epithelium. These are the sperm storage tubules, so called because they can store sperm for long periods of time (10 days to 2 weeks). After an egg is laid, some of these sperm may move out of the tubules into the lumen of the tract, then migrate farther up to fertilize another egg.

Avian sperm

Avian sperm storage tubules


Scanning electron photomicrograph of a 
budgerigar spermatozoon. A indicates 
acrosome; H, head; and T, tail (original 
magnification ×20000) (Samour 2002).

Transmission electron photomicrograph of the longitudinal section of part of the nucleus and midpiece of a budgerigar spermatozoon. N indicates nucleus; PC, proximal centriole; DC, distal centriole; F, axial filament complex; and M, mitochondria (original magnification ×30000) (Samour 2002).

Fertilization of the egg usually occurs in the infundibulum.

Light photomicrograph of a zona-free hamster ovum with
numerous budgerigar spermatozoa bound to its surface
(original magnification ×215) (Samour 2002).

Repelling clingy exes helps snipe save sperm -- Writer Gore Vidal once said that he never passed up an opportunity to have sex or appear on television. Some male birds would disagree on at least one count. Having mated with a female, a male Great Snipe (Gallinago media) will reject her further advances and even chase her away. Male Great Snipe form leks to eye up the talent before choosing a mate. A few males get the most sex. Popular birds can get more than half of the matings, perhaps 10 a day. Hence their pickiness, suggest Saether et al. (2001). As male Great Snipe take no part in caring for their offspring, it was thought they had nothing to lose by mating as much as possible. But top males, overburdened with potential partners, must share sperm with care and spread their favors around. Sperm budgeting is the only possible explanation for male snipes' ungrateful behavior.  Like a nightclub, Great Snipe leks see their share of aggravation. "All four kinds of mating conflicts happen" - male choice, female choice, and male and female competition - explains Saether. Males are more likely to repel clingy exes if there are a lot of other females around. Females fight with one another, and males from neighboring territories chase their rivals' females away. Hostility towards old flames might be a bid to maintain order. "If a male gets rid of an unwanted female it's one less problem to worry about," says Saether. Female snipe probably seek to mate again so that they can get enough sperm to fertilize their eggs. Rejected females tend to lower their sights and settle for less popular males.  -- John Whitfield, Nature Science Update
Photo by Saether et al. (2001)

The Avian Egg

Birds' eggs, like the birds themselves, vary enormously in size. The largest egg from a living bird belongs to the ostrich. It is over 2000 times larger than the smallest egg produced by a hummingbird (see photo to the right; Source: Ostrich eggs are about 180 mm long and 140 mm wide and weigh 1.2 kg. Hummingbird eggs are 13 mm long and 8 mm wide and they weigh only half of a gram. The extinct Elephant Bird from Madagascar produced an egg 7 times larger than that of the Ostrich! Within the egg, three extraembryonic membranes support the life & growth of the embryo:

Egg composition and hatchling phenotype -- Parental investment in eggs and, consequently, in offspring can profoundly influence the phenotype, survival and evolutionary fitness of an organism. Avian eggs are excellent model systems to examine maternal allocation of energy translated through egg size variation. Dzialowski1and Sotherland (2004) used the natural range in Emu (Dromaius novaehollandiae) egg size, from 400 g to >700 g, to examine the influence of maternal investment in eggs on the morphology and physiology of hatchlings. Female Emus provisioned larger eggs with a greater absolute amount of energy, nutrients and water in the yolk and albumen. Variation in maternal investment was reflected in differences in hatchling size, which increased isometrically with egg size. Egg size also influenced the physiology of developing Emu embryos, such that late-term embryonic metabolic rate was positively correlated with egg size and embryos developing in larger eggs consumed more yolk during development. Large eggs produced hatchlings that were both heavier (yolk-free wet and dry mass) and structurally larger (tibiotarsus and culmen lengths) than hatchlings emerging from smaller eggs. As with many other precocial birds, larger hatchlings also contained more water, which was reflected in a greater blood volume. Emu maternal investment in offspring, measured by egg size and composition, is significantly correlated with the morphology and physiology of hatchlings and, in turn, may influence the success of these organisms during the first days of the juvenile stage.

Emu egg & embryo

Yolk contains maternal antibodies -- Antibodies are deposited in eggs during yolk formation through the deposition of immunoglobulins, primarily IgY (also called IgG), in the yolk. In Chickens (Gallus domesticus), maternal IgY is catabolized by offspring over the first 14 days post-hatching and, by about 5 days post-hatching, offspring begin to synthesize their own IgY. As a result, after approximately two weeks the circulating IgY in young is principally of endogenous origin. Adult levels are attained between six weeks and six months of age. However, maternal antibodies may continue to affect offspring phenotype even after they are catabolized by influencing growth and developmental rates. In the absence of maternal IgY in chickens (due to surgical bursectomy of the mother during her own embryogenesis), the number of cells in the spleen that help lymphocytes (helper T cells) attack antigens (foreign proteins on pathogens) is depressed. Also, the immune responsiveness of offspring is depressed, which could lower the survival of offspring particularly in harsh disease environments (Grindstaff et al. 2003).

Antibodies 'attack' pathogens or toxins they
produce by binding to antigens (e.g., proteins
in the membranes of bacteria) via their  'binding 
sites' (the black areas above). This binding 
can neutralize toxins and attract white blood 
cells that eliminate pathogens (by phagocytosis).

The familiar color of a chicken’s egg yolk (a) is in stark contrast to the richly pigmented egg yolk of a lesser Black-backed Gull, Larus fuscus (b). Such high maternal investment of carotenoids into egg yolk is typical among wild bird species, suggesting that these biologically active pigments serve important functions in the developing bird
(From: Blount et al. 2000).

Why egg yolk is yellow (or red) (Blount et al. 2000) -- Egg yolk in birds is colored yellowish-red by carotenoids. Until recently, there has been no adaptive explanation of why many egg-laying animals provision their eggs so richly with carotenoids. It now appears that, in developing birds, carotenoids protect vulnerable tissues against damage caused by free radicals. Athough embryonic tissues depend on oxidizable, unsaturated fatty acids in yolk, their abundance makes the tissues susceptible to peroxidation caused by reactive oxidative metabolites and by free radicals, which are produced as normal by-products of metabolism. Protection against lipid peroxidation in young birds is afforded by the actions of yolk-derived carotenoids and other antioxidants, like vitamin E. Antioxidants also protect passively-acquired antibodies (IgY; see above) against break-down. Thus, maternal investment in egg composition, including carotenoids, might have a greater influence on offspring viability than has been realized. The use of carotenoid pigments in the sexual displays of female birds might indicate their ability to produce high quality eggs and chicks.

Keratin fibers from the outer shell membrane can be seen above, attached to the
calcium carbonate crystals that make up the main shell structure.

Thousands of tiny pores like the one pictured above, cover the shell, providing a passage for gas exchange.

Weaker Birds Use Steroids to Boost Offspring -- Verboven et al. (2003) reported that female gulls in poor condition were more likely to give their chicks a hormone boost to improve their chances of survival. Verboven and her colleagues experimentally enhanced maternal condition by supplementary feeding Lesser Black-backed Gulls (Larus fuscus) during egg formation and compared the concentrations of steroids (including testosterone) in their eggs with those in eggs laid by control females. Egg androgens could affect offspring performance directly through chick development and/or indirectly through changes in the competitive ability of a chick relative to its siblings. Contrary to expectation, females with experimentally enhanced body condition laid eggs with lower levels of androgens. This suggests that less healthy females pass on more steroids than healthy ones in a bid to enhance the performance of their young. Verboven noted that  “We originally thought that gulls in good condition would put more steroids in their eggs. But we discovered that healthy birds don’t tend to give their eggs the extra boost.” She compared the situation to struggling athletes who take performance-enhancing drugs. She added: “A poor sports person maybe wants to use steroids to conceal poor performance but if you are good you don’t need to use them.”

Lesser Black-backed Gulls
© Arthur Grosset 

The Ecology of Egg Colors (see Birds: A Virtual Exhibition - The Provincial Museum of Alberta)

Egg colors and markings have strong adaptive values. Originally, birds' eggs were probably all white, as reptile eggs are. Eggs that are laid on the ground or in open nests in trees, rather than in cavities, often exhibit cryptic coloration. The eggs blend in with their surroundings and are much less visible to potential predators (e.g., a Killdeer nest).

Sometimes eggs that are laid in open nests are white at first. They then become stained by the mud and rotting vegetation in the nest. Grebes lay white eggs that become stained and cryptically colored over time.

Red-necked Grebe nest

In some species, such as the Common Murre, where different females lay eggs with very different markings, the uniqueness may have a purpose. Distinctive patterns, as in the eggs shown below, help females identify their own egg in a colony where thousands of eggs may dot a cliff face.


Eggs of kingfishers and other cavity nesting birds, such as woodpeckers and some owls, are often white. The brightness of the eggs may help the parents to more easily locate them in the cavity. Shown here is the egg of a Barn Owl.




Female birds turns part of the cloaca and the last segment of the oviduct inside out ("like a glove"). The vent is then everted and the egg emerges far outside at the end of the bulge. As a result, the egg does not contact the walls of the cloaca and get contaminated by feces. In addition, the intestine and inner part of the cloaca are kept shut by the emerging egg, and their contents cannot leave when the hen strains to deliver the egg. Therefore, eggs are always clean when laid (van der Molen 2002).

Literature Cited:

Akins, C. & M. Burns. 2001. Visual control of sexual behavior. In R. G. Cook  (Ed.), Avian visual cognition [On-line]. Available:

Blount, J. D., D. C. Houston, and A. P. Møller. 2000. Why egg yolk is yellow. Trends in Ecology and Evolution 15: 47-49.

Brown, C.R. and M. B. Brown. 2003. Testis size increases with colony size in cliff swallows. Behavioral Ecology 14:569-575.

Dzialowskil, E. M. and P. R. Sotherland. 2004. Maternal effects of egg size on emu Dromaius novaehollandiae egg composition and hatchling phenotype. J. Exp. Biol. 207:597-606.

Ellegren, H. 2001. Hens, cocks and avian sex determination: a quest for genes on Z or W? European Molecular Biology Organization Reports 2:192-196.

Grindstaff, J. L., E. D. Brodie III, and E. D. Ketterson. 2003. Immune function across generations: integrating mechanism and evolutionary process in maternal antibody transmission. Proc. Royal Soc. Lond. B 270: 2309-2319.

Pettingill, O.S., Jr. 1985. Ornithology in Laboratory and Field, Fifth ed. Academic Press, New York, NY.

Rosenzweig, M.R., A.L. Leiman and S.M. Breedlove. 1996. Biological Psychology. Sinauer Associates, Sunderland, MA.

Rutstein, A.N.,  P. J. B. Slater, and  J. A. Graves. 2004. Diet quality and resource allocation in the zebra finch. Proc. R. Soc. Lond.  B (Suppl.). Published online, 20 February 2004.

Saether, S. A., P. Fiske, & J. A. Kalas. 2001. Male mate choice, sexual conflict and strategic allocation of copulations in a lekking bird. Proceedings of the Royal Society London B 268: 2097 - 2102.

Samour, Jaime H. 2002. The Reproductive Biology of the Budgerigar (Melopsittacus undulatus): Semen Preservation Techniques and Artificial Insemination Procedures. Journal of Avian Medicine and Surgery 16: 39-49.

Sato, T., Yen-nien Cheng, Xiao-chun Wu, D. K. Zelenitsky, & Yu-fu Hsiao. 2005. A Pair of Shelled Eggs Inside A Female Dinosaur. Science 308:375.

van der Molen, W. H. 2002. Laying an egg.

Verboven, N., P. Monaghan, D.M. Evans, H. Schwabl, N. Evans, C. Whitelaw, and R.G. Nager. 2003. Maternal condition, yolk androgens and offspring performance: a supplemental feeding experiment in the Lesser Black-backed Gull (Larus fuscus). Proceedings of the Royal Society: Biological Sciences 270: 2223 - 2232.

Welty, J.C. and L. Baptista. 1988. The life of birds, fourth ed. Saunders College Publishing, New York, NY.

Winterbotton, M., T. Burke, & T. R. Birkhead. 2001. The phalloid organ, orgasm and sperm competition in a polygandrous bird: the red-billed buffalo weaver. Behavioural Ecology and Sociobiology 50: 474-482.

Useful Links:

Eggs, Nests, and Predators

Feathered Nests

Form and Function - A Closer Look at the Chicken Egg Shell

Hormonal Control of Incubation/Brooding Behavior: Lessons from Wild Birds

Incubation: Heating Egg

Incubation Time


Nest Building

Nest Lining

Nest Materials

Nest Sanitation

Parental Care

Precocial and Altricial Young

Who Incubates?

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