Avian Reproductive System

Sonia M. Hernandez

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?

Objectives

Paired testes in males Ovary Testes & follicles increase

dramatically in size as the breeding season approaches. As day length increases Stimulation of hormone

secretion

Gonads

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

Testes

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)

Ovary

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.

The egg’s journey

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

Copulation

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

Copulation

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.

Sperm storage

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.

The fate of sperm

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)

The egg

Eggs consists of 4 primary components: yolk

energy-rich supply of food 21 - 36% lipids & 16 - 22% 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

Egg components

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)

The egg

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

Egg content linked to offspring lifestyle

amount of energy available to the developing embryo

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)

Egg color

White-cavity nesters Blue-low light Pigment-female quality or

Ca2+ deficiency Predation? Brood parasitism?

Many colors, many reasons

Why blue eggs in Eastern bluebirds?Female quality?Biliverdin….

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).

Egg layinghttp://www.youtube.com/watch?v=RAiTOuSyJsQ

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.

Sex determination

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.

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

Scrape nests Shorebirds,

nighthawks, gulls, penguins, B vultures

Burrow nests Kingfishers, bank

swallows, motmot

Nest Types

Cavity nests Woodpeckers,

hornbills, waterfowl, owls

Platform nests Grebes, loons

Nest Types

Cupped nests Hummingbirds, many

passerines Suspended Pendulous adherent Grebes, loons

Nest Types

Cupped nests Suspended Pendulous Adherent

Nest Types

Fragrances Blue tits add lavender, yarrow, curry, mint etc Antiparasitic/antimicrobial, insecticidal

material Scat

Ward off predation? Preen waxes

Now add the finishing touches!

Architectural feats! Bald eagles

Clay cups Ovenbirds http://

www.youtube.com/watch?v=PSJsPxAfapA

Special nests!