So far, in studying
reproductive life cycle considerations, we have examined puberty, pregnancy and
parturition in mammalian species in which the offspring develop inside the body of the dam
and, after birth, are provided nourishment in the form of milk produced by the dam.
In the avian species, however, the dam produces an egg in which, if fertilized, the
offspring will undergo growth and development outside the body of the
dam. The dam does not suckle it's young, either. As a consequence, there are a
number of contrasts between avian and mammalian reproductive life cycles.
Puberty and Breeding in the Avian
- The hen reaches puberty and starts to
produce eggs at 4-5 months of age. As with mammals, the reproductive system is not
functioning completely normally at the onset and hens at puberty produce small egg sizes
and high percentages of eggs with twin yolks. Since these eggs do not produce viable
offspring, hens are not bred to produce young until they reach 5-6 months of age.
- The cockerel is capable of insemination at 4-5
months of age, but, like the hen, is not used for breeding until 6 months of age to insure
- The natural instinct is to lay a "clutch"
of eggs, become "broody", stop laying eggs (ovulating), and set on eggs to
hatch. This broodiness has, for the most part, been bred out of our commercial
breeds of poultry, and hens will produce eggs continuously. Hens do not have an
estrus cycle, and will lay an egg nearly every day.
- In contrast with mammalian species in which both
ovaries are functional and either ovulate simultaneously, or alternately, 99% of hens have
only one functional ovary.
Reproductive Physiology of the Hen
tract of the hen is also different from mammals, and different functions are performed in
different segments of the tract. The major structures are as follows:
- OVARY - containing immature and mature
follicles. The mature follicles consist of the egg "yolk" and the
- INFUNDIBULUM - yolk with attached
ovum is snatched up by the infundibulum. It is at this point in the reproductive
tract that the ovum is fertilized if the hen has been mated with a cockerel.
Spermatozoa from the cockerel are stored in "sperm nests" located within the
infundibulum and are capable of fertilizing ova for up to 30 days after mating.
- MAGNUM - while traveling through this
part of the oviduct, the albumin or egg white is formed.
- ISTHMUS - the tough outer membrane located
just beneath the egg shell is formed in this part of the oviduct.
- UTERUS - also referred to as the "shell
gland", this is where the egg shell is formed. Most of the transit time from
ovulation until the egg is laid is spent in the uterus.
- VAGINA - the egg travels through the vagina
into the cloaca, from which it is "laid."
- CLOACA - this is the common external opening
from which the contents of the urinary tract (urates), the intestinal tract (feces) and
the reproductive tract (eggs) exit the hen.
This image was taken from the Purdue Avian Sciences web page. For
additional pictures of the reproductive tract of the hen, check the University of Georgia,
Department of Poultry Sciences, PS
202 lab review materials. Slides 68, 70, 71, 73, 77 and 84 are particularly good
- A hen is capable of producing an egg every 25 hours.
- Eggs are produced and laid regardless of whether the
hen has been mated and the eggs are fertile or not.
- A hen is capable of laying approximately 270 eggs per
- The embryo in a cracked fertile egg will not develop.
- Incubation and hatching of fertile egg
- humidity & temperature control are important
factors in the hatchability of fertilized eggs.
- Chicken -- eggs incubate 21 days; spend 1 day in hen;
22 days from fertilization to birth of chick
- Turkey, duck -- eggs incubate 28 days for a total of
29 days from fertilization to birth.
In addition to differences in the reproductive physiology, the development of avian
offspring differs significantly from mammals in a number of other ways:
- The shell takes the place of the uterus or womb in
providing a dark, warm, moist sterile environment in which the offspring develop.
- The albumin or egg white serves as a shock absorber
for the developing embryo, just as the amniotic fluid does within the mammalian amniotic
membranes (birth sac).
- The yolk provides nourishment to the developing chick
just as the umbilical cord provides nourishment to the developing mammal. The yolk
also provides maternal antibodies to the chick to protect against infectious agents in the
environment to which the hen has developed an immunity (this is similar to colostrum in
- The chick develops outside and independent of the
hen, and does not need the hen for survival provided that the proper environment is
provided by man for incubation of the egg. Mammals develop inside the uterus of the
dam and are dependent upon the health and well-being of the dam throughout the entire
gestation for their health and well-being.
[home] [next] [previous] [lec notes]