Continued from page 5Hatching
Throughout the development of the embryo, the egg steadily loses water by transpiration through the chorioallantoic membrane. This water loss is a linear and consistent event throughout the entire incubation process. Because of this loss of water and the loss of yolk fats metabolized during development, the egg is much lighter at hatching than when it was laid. In most avian species, the weight loss is approximately 15% of the original weight of the egg. The eggshell, too, is thinner than when it was laid because the chick has absorbed much of the calcium from the inner shell lining. In the imminent hatching period, the chick absorbs the remainder of the yolk sac into the abdominal cavity and also begins to swallow any remaining amniotic fluid that remains.
Two specialized structures found only in hatchlings aid the chick in its struggle to break open the shell. A small, sharp "egg tooth" develops on the dorsal tip of the rhinotheca and is used to "pip" and "cut" out of the shell. A substantial enlargement of the complexus muscle (pipping muscle) in the proximal dorsal cervical region helps to brace the neck and cushion the head as the chick forces the egg tooth through the shell. After hatching, most of the fluid within the complexus muscle is reabsorbed, and this muscle continues to function as an extender of the head in most adult birds. The egg tooth is lost in the first few weeks following hatching.
Immediately prior to the start of the hatching process, the air cell expands to encompass approximately 20-30% of the total internal egg volume. This change can be noted by candling, and is termed the "draw down". At the start of the hatching process, the beak of the embryo penetrates the inner shell membrane where it forms the inner wall of the air cell, and the lungs become functional by inspiring air from the air cell. An increase in plasma CO2 is associated with the trigger for the internal pipping process. The external pip occurs when the chick cracks or cuts the outer shell membrane and shell. Again, rising CO2 in the air cell chamber serves as the trigger for the spastic neck and associated muscle contractions involved in the hatching process. In the domestic fowl a period of about 20 hours elapses between pipping and hatching. In non-domestic bird species, this time frame from internal pipping to hatch can normally range from six to up to 48 hours. Most parrot species normally hatch within a 24 hour period from internal pip to completed hatch.
Problems with the Hatch
The following charts are provided to furnish basic information that will allow for trouble shooting problem hatches. Keep in mind that the actions taken are based on overall incidences of problems, rather than isolated incidences. From a flock perspective, take caution about recommending that we throw out the baby with the wash!
Malposition Classification and Descriptions (poultry):
Malposition I: Head is between the thighs (position assumed immediately prior to hatching position).
Malposition II: Head is in the small end of the egg. Approximately 50% of the embryos can hatch from this position if they do not suffocate. Manual intervention may help reduce mortality rates associated with this position.
Malposition III: Head is toward or under the left wing instead of the right wing. This is a lethal position as the embryo tends to rotate counterclockwise to hatch which it cannot do in this position.
Malposition IV: The embryo is rotated with the beak away from the air cell. It is impossible to successfully hatch from this position, although the chick will often pip the shell and frequently pip a vessel and cause hemorrhage from the chorioallantoic vessels.
Malposition V: Chicks have their feet over their head. This position makes rotation very difficult and the embryo usually fails to hatch. This is an uncommon malposition in psittacine birds.
Malposition VI: The beak is above the right wing. This is a nonlethal variant of normal positioning.
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