Hatching day is my most favorite day. At the point of hatching, I have been gently tending my eggs for 21 days, ensuring that the temperature and humidity are just right (consistent and stable), making sure the eggs get turned at least once an hour, candling them to see the developmental progress of the chicks inside, and ensuring that when hatch day comes they have the best shot at life that I can offer them. And, of course, talking to them, you know, just in case they can hear and learn to recognize my voice. The excitement is always palpable and I have yet to grow tired of watching an unblemished egg turn out a wet, tired chick. So for those of you who haven’t experienced this, here’s what happens.
On day 18 of incubation, I increase the humidity of the incubator. This allows for more moisture to pass through the shell and helps the chick turn into position in his cramped quarters. I also stop rotating the eggs at this time. While rotating the egg in the first 18 days of incubation is helpful to prevent adhesions of the extra embryonic membranes to the shell and allow for proper development of the chick, the egg has to rest the last few days of incubation so the chick can position itself for hatching with its head toward the fat end of the egg. Why?, you ask? Because that is where the air sac is. Over the last 21 days, air has been moving through the shell, some air gets trapped between the hard external shell and the internal soft shell, this creates a pocket of air. As day 21 rolls around, the chick pokes its head out from under its wing and punctures the internal soft shell, giving him access to the pocket of air that has been accumulating. The next time you crack open a store bought egg, look in the fat end of the empty shell and you’ll likely see this air sac.This event is known as the internal pip. This pocket of air gives the chick oxygen to breathe as the remainder of the yolk and blood vessels in the extra embryonic membranes gets absorbed, as those were its previous source of oxygen. Getting into that position was tough, hard work and the chick will rest for a bit. It’s hard to tell how long the chick rests after the internal pip, unless you’re candling consistently during this process, but it’s best to leave the chick to rest.
The first sign you’ll see of hatching, besides a shaking egg as the chick moves around inside, is a small crack in the egg shell. This cracking of the external, hard shell is known as the external pip. Once this happens you will be able to hear the chick chirping. The external shell contains lots of calcium and is super tough to crack. The chick has some help with this as his beak has a temporary tip on it, called the egg tooth, that is hard and sharp and assists with breaking through the shell. The chick will lose this in the days after hatching. Again, the chick will rest after this pip. Often up to 24 hours. It is important to be patient at this point and let the chick do the work. Cracking the shell, tearing the membranes, and trying to get the chick out can result in injury and/or death of the chick. Let the chick get itself out of the egg. These rest periods serve another important purpose: they allow the chick to fully absorb the remainder of the yolk and the blood in the blood vessels of the surrounding tissues. The contents of these things allows the chick to survive until it is able to learn to eat at around 2 to 3 days old.
Eventually, you will see the crack in the egg start to move. That’s the chick pecking at the crack and trying to open it more. Bits and pieces of the shell will fall away as the chick makes the crack bigger. The chick will then rotate himself one direction or the other and continue cracking the egg open. This will look like a line going around the outside of the egg. This line is called the zip line. When the zip line is three-quarters around the shell, the chick will start stretching and trying to pry open the now two parts of the shell. The chick will continue to struggle stretching out his legs and trying to push himself out of the shell. Eventually, the shell will give way, breaking in two and the exhausted, wet chick will lie in the warm brooder until he has regained enough strength to start exploring his new environment.
The video below demonstrates all of these steps and points out the zip line.