Avian Reproductive System

by Sherri Carpenter
Summer 2003

The Avian Reproductive System

From cell to egg to a baby bird, is an amazing process. With the unity of a sperm cell to an ovum, a single cell develops into an embryo and a new life begins. I will begin with the reproductive organs.

Male Reproductive Organs

male reproductive systemMale birds have a pair of testes that resemble a bean shape and each are located in front of the top lobe of the kidneys. During non-breeding season the testes are difficult to locate due to their small size, but during breeding season they may grow as much as several hundred times their non-breeding size.

As in mammals, the sperm cells of birds cannot develop fully at high temperatures that are found within the body cavity. Some birds experience a nightly drop in body temperature that allows the sperm cells to develop, while other birds have a swelling at the end of the tube (vas deferens). This tube connects the testes to the cloaca, and functions like a mammals scrotum holding the sperm away from the higher body temperatures that are within the abdomen.

Most bird species rub their cloacal areas together to transfer the male's sperm but ostriches, rheas, strokes, flamingos, ducks and a few other families actually have an erectile grooved penis on the back wall of the cloaca to transfer sperm.

Female Reproductive Organs

female reproductive systemThe female organs consist of the ovary and the oviduct that leads to the cloaca. With most bird species, the ovary is on the left side with the right side being underdeveloped and nonfunctional. It is thought that being only one sided reduces body weight and eliminates the possibility of carry two large, fragile eggs in the abdomen cavity at the same time.

The ovary when mature looks like a cluster of grapes. It may contain up to 4,000 small ova that can develop into yolks. Yolk protein, lipids and fats are manufactured in the liver and travel through the bloodstream to the immature ovum, during the maturation stage. Each yolk is attached to the ovary by a thin membrane sac or follicle having a fine network of blood vessels. The germinal disc of a developing yolk contains the single ovum cell which, after fertilization develops into a chick. The ovary enlarges during breeding season as much as fifty times its non-breeding weight.

The oviduct is a large coiled tube where all parts of the egg are formed except the yolk. It consists of several sections. Starting from the top: ostium, infundibular funnel, magnum area, isthmus, uterus and the vagina. They each play a part in the development of the egg.

When the brain’s pituitary gland releases a luteinizing hormone, LH, ovulation begins. The sac around the yolk ruptures and releases the yolk from the follicle, the yolk is kept intact by a fine membrane called the vitelline. The yolk is than engulfed by the infundibulum, with its thin, funnel-like lips. If the infundibulum cannot pick up the yolk, it is usually absorbed by the abdominal cavity or can cause peritonitis, an inflammtion of the lining of the abdominal cavity. Once inside the infundibulum fertilization can begin. The sperm have been stored in glands or nests, located in the infundibulum and are released as the yolk passes by. The sperm cell must penetrate the thin vitelline membrane and reach the female cell to complete fertilization.

The yolk than travel immediately into the magnum section for an stay of approximately three hour. In this time the egg white is added to the ovum. It is a protein substance containing mucin, globulin and albumen including sodium, magnesium and calcium. This serves as a shock absorber and feed the developing embryo.

In the isthmus the stay is approximately 75 minutes and another 10% of the albumen, the chalazae and shell membranes are added to the ovum. The chalazae are little ropes that attach to the yolk to keep it in the center and allow the yolk to rotate the keep the germinal disc ( the fertilized zygote) on top. The shell membranes are joined except in the area where the air cell will be, where they will separate.

The uterus is a thick-wall membrane where the egg will spend 20 hours. Through the shell membranes, water and salts are added which plump out the egg. The shell is added here and has three layers. The inner layer is first produced and called the mammillary layer. The middle layer is called testar and is the thickest and the outer layer is made of dried mucous. The shell is composed of calcium carbonate.

In the vagina the stay is short. A coating called “bloom” is added to keep harmful bacteria and or dust from entering the egg. The soft egg is shaped in the vagina depending on the shape of the bony pelvis.

The egg than travels to the cloaca and is laid. When the egg is laid, it is the same temperature as the hen and fills the shell. As it cools, it looses volume and the density changes slightly, creating a pressure, which draws air into the egg and forms the air cell. The shell hardens as it cools and dries. This whole process from start to finish has taken approximately 24 hours.

A brood spot develops on the hen and can develop on some males. The temperature rises on the brood spot due to a large amount of blood collecting in that area. Incubation of the eggs is usually 18 to 29 days depending on species. About 15% of an eggs original weight is lost due to evaporation in the incubation process. The eggs incubate at 99.1 to 99.5 degrees Fahrenheit with humidity between 50 and 53%. I will go into the embryo development in the next article.