1. Avian Reproductive System
Sonia M. Hernandez

2. Objectives
Know the names of the major reproductive organs and their function
Be able to trace the path of an egg through the reproductive tract and how it develops
Understand bird sex
What are the components of bird eggs? What function do they serve?
Why are eggs the color they are?
What are the different types of nests and who builds them?

3. Gonads Paired testes in males Ovary
Testes & follicles increase dramatically in size as the breeding season approaches.
As day length increases
Stimulation of hormone secretion
photic stimulation of the hypothalamus results in the secretion of Gonadotropin releasing hormone (GnRH below). When activated by GnRH, the anterior pituitary secretes two gonadotropin hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH acts on sperm-producing structures in the testes, while LH acts on the interstitial cells of the testes causing them to secrete the steroid hormone testosterone. The pituitary gland monitors the amount of testosterone in the blood, thus creating a negative feedback loop to maintain hormone levels within a set range (Akins and Burns 2001).
Ambient visual cues, such as daylight, activate photosensitive loci in the brain both indirectly, through the eyes, and directly, through the skull. The hypothalamus of the bird brain contains special cells that are sensitive to extremely low light levels, intensities comparable to the amount of light that can penetrate brain tissue (Akins and Burns 2001).

4. Testes
Paired abdominal testes lying cranioventral to the first kidney lobe. The vas deferens emerges medially and passes caudally to the cloaca where it has a common opening with the ureter in the Urodeum.
As in mammals, sperm formation is temperature sensitive, and maturation is assisted by nocturnal drops in temperature 
Have relatively low extragonadal sperm reserves and sperm are ejaculated soon after production in the testes

5. Ovary
Most only LEFT ovary and oviduct. In some birds, such as hawks, the right ovary and oviduct also develop. A mature ovary looks like a cluster of grapes and may contain up to 4,000 small ova which can develop into mature ova
With fertilization, the ovum (egg) becomes a developing embryo
The embryo passes through the oviduct; typically takes about 24 hours (for passerines & most other birds)
The demand for calcium to make the egg shell is very high, and so the circulating levels of blood calcium in birds are greatly elevated compared to mammals (2X)

6. The egg’s journey
Ovulation results in the release of an egg from a mature follicle on the surface of the ovary. The egg is picked up by the infundibulum and ciliary currents carry it into the magnum region. In 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.

7. Copulation
For most birds, copulation involves a 'cloacal kiss', with the male on the female's back & twisting his tail under the female's

8. Copulation
males in a few species, including most waterfowl & ostriches, have an intromittent organ

9. Sperm storage
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, -can store sperm for long periods of time (10 days- 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.

10. The fate of sperm
On ovulation, the sperm swarm over the surface of the ovum; their target is the germinal disc, which contains the female pronucleus.
Polyspermy is typical in birds. Several sperm enter the germinal disc region
However, only a single spermatozoon fuses with the female pronucleus and the remaining sperm are shifted to the periphery of the germinal disc and play no further part in development.

11. The 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
Female kiwis produce extremely large eggs for their size (with substantial amounts of yolk)

12. Egg components Eggs consists of 4 primary components: yolk albumen
energy-rich supply of food
% lipids & % proteins (with the rest being water)
the yolk is suspended in the center of the egg by twisted strands of protein fibers called chalazae
albumen
90% water & 10% protein
the embryo's water supply, but also serves as a 'shock-absorber' to help protect the embryo
buffers embryo from sudden changes in temperature
shell membranes
inner and outer shell membranes. They protect the egg from bacterial invasion and help prevent rapid evaporation of moisture from the egg.
shell
protects the embryo
contains thousands of pores that permit gas exchange
generally white in cavity-nesters & colored and patterned in open nesters

13. The egg
amnion
surrounds only the embryo
in which the embryo floats; fluid keeps the embryo from drying out and protects it
chorion - surrounds all embryonic structures & serves as a protective membrane
allantois (or allantoic sac)
grows larger as embryo grows, fuses with the chorion & is called the chorio-allantoic membrane
works together with chorion to permit respiration (exchange of oxygen and carbon dioxide) and excretion
important in storage of nitrogenous wastes (uric acid)

14. Egg content linked to offspring lifestyle
amount of energy available
to the developing embryo
From top to bottom, altricial Brown Creeper, a semiprecocial Least Tern, a precocial Ruddy Duck, a superprecocial Mallee Fowl and a Brown Kiwi (Apteryx australis). Kiwis are ‘outliers.’, bc young typically remain in the nest for several days and so are semiprecocial

15. Egg color
Originally, birds' eggs were probably all white, as reptile eggs are. Eggs that are laid on the ground often exhibit cryptic coloration.
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
Some eggs are patterened bc it helps females find their eggs in colonies (Common murre)

16. Many colors, many reasons
White-cavity nesters
Blue-low light
Pigment-female quality or Ca2+ deficiency
Predation?
Brood parasitism?
Several species of birds have blue eggs, and David Lack (1958) suggested that, in habitats where light levels are low, blue eggs might be cryptic. If true, that could help explain the blue eggs of some open-cup nesting birds that occur in forest habitats such as Wood Thrushes. However, Lack’s hypothesis cannot explain why some birds that nest in cavities, like European Starlings and Eastern Bluebirds, also have blue eggs. One hypothesis is that the blue-green color of eggshells represents a signal of female quality to their mates ( Moreno and Osorno 2003). The pigment responsible for the blue-green color is biliverdin, a substance produced when the hemoglobin of damaged red blood cells is catabolized and also known to have strong antioxidant properties. Antioxidants are important because they can convert free radicals, molecules that can damage DNA, proteins, and other macromolecules, into less reactive substances. Deposition of this pigment in eggshells by laying females may, therefore, signal their capacity to produce antioxidants and control free radicals. Male birds paired to females of such quality that they are able to deposit antioxidants in eggshells rather than retaining them may then expend greater effort in caring for their superior offspring (Kilner 2006). In support of this hypothesis, the provisioning rates of male Pied Flycatchers (Ficedula hypoleuca) and the intensity of the blue coloration of eggs were found to be positively correlated (Moreno et al. 2004). Also, female Eastern Bluebirds in better body condition were found to lay more colorful eggs, supporting the hypothesis that biliverdin pigmentation in eggshells reflects female condition (Siefferman et al. 2006).
Why blue eggs in Eastern bluebirds?
Female quality?
Biliverdin….

17. Egg laying
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).

18. Sex determination
In mammals the female is XX and the male is XY, this results in the male sperm determining the sex of the offspring. In birds it is the opposite. The female is WZ and the male is ZZ. All avian sperm is Z, and the female releases either a Z egg or W egg. Therefore it is the female bird that determines what sex her offspring will be.

19. Although common in reptiles, incubation temperature has not been considered to be a factor in determining sex ratios in birds. Goth and Booth (2005) found that incubation temperature does affect sex ratios in megapodes, which are exceptional among birds because they use environmental heat sources for incubation. In the Australian Brush-turkey (Alectura lathami), a mound-building megapode, more males hatch at low incubation temperatures and more females hatch at high temperatures, whereas the proportion is 1:1 at the average temperature found in natural mounds. Chicks from lower temperatures weigh less, which probably affects offspring survival, but are not smaller.
Goth and Booth (2005) suggest a sex -biased temperature-sensitive embryo mortality because mortality was greater at the lower and higher temperatures, and minimal at the middle temperature where the sex ratio was 1:1.

20. Nest complexity
Bird nests vary from a simple accumulation of materials on the ground to elaborate refuges in or on secluded & elevated substrates. Dial (2003) observed that nest construction and placement are correlated with other features such as flight ability

21. Nest Types Scrape nests Burrow nests
Shorebirds, nighthawks, gulls, penguins, B vultures
Burrow nests
Kingfishers, bank swallows, motmot

22. Nest Types Cavity nests Platform nests
Woodpeckers, hornbills, waterfowl, owls
Platform nests
Grebes, loons

23. Nest Types Cupped nests Hummingbirds, many passerines Suspended
Pendulous
adherent
Grebes, loons

24. Nest Types
Cupped nests
Suspended
Pendulous
Adherent

25. Now add the finishing touches!
Fragrances
Blue tits add lavender, yarrow, curry, mint etc
Antiparasitic/antimicrobial, insecticidal material
Scat
Ward off predation?
Preen waxes

26. Special nests! Architectural feats! Clay cups Bald eagles Ovenbirds