When a cockatiel egg is ready to hatch the chick undergoes
different chemical changes within the body due to different conditions that
occur inside the egg. Below is a description of the hatching process and a
series of pictures I was lucky enough to get of one of my eggs hatching.
Egg hatching normally begins approximately 16 - 21 days
after the initial incubation by the parents commences. During this
incubation period the chick has grown to look like the chick we recognise
and through that growth has continually used resources within the egg and
also released waste products. The egg has lost weight compared to the weight
when it was laid due to loss of water through the membrane and also due to
the chick metabolising fats from the yolk. The chick also absorbs calcium
from the inner lining of the shell making it thinner and lighter in weight.
Just before it hatches the chick will also draw up the remaining yolk sac
into its abdomen and orally take in any fluid that remains inside the shell.
All these factors can mean a reduction of around 10% in the weight of the
egg over the total incubation period.
Just prior to hatching the air sac in the larger rounded
end of the egg will appear enlarged and occupy 20 - 30% of the volume of the
egg. The tooth like prong on the end of the beak that the chick has developed
to assist in hatching pierces the air sac and causes this enlargement and the
chick takes its first breath of air. This process of the air sac becoming
bigger is called the "draw down" process and can be seen when candling.
Because the inside of the egg is still not open to the outside air the chick
causes an increase in the carbon dioxide level within the air sac as it
breathes. The rise in CO2 causes muscles to contract in the abdomen and thus
the yolk sac gets drawn into the body. This rise in carbon dioxide also causes
twitching or contracting of the muscle at the back of the neck that is
enlarged at this stage to help with the pipping process. This contracting
causes the tooth like prong or egg-tooth to pierce the inner shell membrane
and create what we see as the first pip in the shell.
The contracting neck muscles makes the head move in a
jerking motion. Along with causing the egg-tooth to break through the shell it
also causes the chick to rotate with in the shell. This creates a line of pip
marks around the end of the shell and gives almost like a separate top to the
egg that the chick can then push off. The moisture retained within the shell
membrane is used as a lubricant to allow the chick to rotate and eventually
break free of the shell.
This complete process from the initial draw down of the air
sac to hatching normally takes from 24 - 48 hours. Anything longer than this
usually means there is a problem and an assist hatch may be necessary. If
everything appears normal then it is best not to intervene until it becomes
absolutely necessary. I will cover the subject of assist hatches and hatching
problems in another article.
Here is a series of pictures I took a while ago of a
chick in the process of hatching. From the first picture to the last took
approximately 15 minutes. During the final stages the gap in the shell would
open and then close as the baby pushed and then rested.