How do chicks breathe inside an egg?

The miracle of life is truly fascinating – no matter if you are talking about a full human growing from a clump of cells or tiny chick! So what exactly goes on inside that egg during the short 21 days that it takes for a chick to hatch?

Chicken embryos were among the first & most often studied embryos thanks to their easy availability. Thankfully for us, that means there is a lot of information out there on what goes on inside that egg.

Can any egg hatch under the right conditions?

No, in order for an egg to hatch it must be fertilized by a rooster. If you do not have a rooster there is no chance of a chick developing. Click here to read more about the difference between fertilized & unfertilized eggs

How exactly does an egg form?

Just like human females, a hen is born with all the eggs she will ever have inside her ovaries. The process all begins with a tiny ovum in the hen’s ovary. The ovum matures into the yolk of the egg. Once an ovum is ready, it will be released into a “holding tank” called the infundibulum for about 15 minutes.  

If the ovum is to be fertilized, it needs to happen in this short 15 minute window.  Hen’s bodies can store a rooster’s sperm for up to a week after mating, to be used during this window. So it’s a good bet that if your hen has been mated within a week that her eggs will be fertilized.

If you look closely at an unfertilized egg yolk you will see a tiny white spot. That spot is called the blastodisc, the female cell. If a rooster has mated the hen, his sperm will mix with the cell making embryonic development possible with incubation. Even a fertilized egg won’t develop unless it is incubated.

Fertilized or not, the yolk will continue through the oviduct where the egg white is added. A thin membrane keeps everything loosely together.

Chalazae are added to keep the yolk centered in the egg. “Ropes” of protein, they act sort of like bungee cords to protect the yolk from hitting the sides. This “rope” can still be seen in farm fresh eggs (it has usually dissolved by the time they get to grocery stores).

By now the egg has reached the hen’s uterus. In the uterus the shell gland forms the hard egg shell. This process can take over 20 hours. All egg shells start out as white, but some breeds during the final stages add a color to the shell making process! The final stage before the egg is laid is to add a thin protective coating called a bloom to the egg. This step helps keep bacteria out of the egg.

Will my fertilized eggs start to develop sitting in the nest boxes?

Not unless they have a hen sitting on them to incubate them. Chicken embryos need a temperature between 99-102 degrees and a humidity level of 50-65% to develop. So I suppose in theory if the outdoor temperature & humidity stayed consistently day & night in those ranges for 21 days the eggs could hatch on their own.

Since that is basically impossible, a nice warm chicken butt is needed! A brooding hen will pluck out her underside feathers, rotate the eggs and take breaks to instinctively keep her eggs at the right temperature.

My chicken only lays one egg per day – how can she possibly hatch a clutch of multiple chicks? Won’t they all hatch at different times?

This is the beauty of the bloom added in the last stage of egg development. The bloom is a protective coating that keeps bacteria from spoiling the eggs. This allows a hen to gather a clutch over the course of 7-10 days without compromising the health or fertility of the eggs. The eggs won’t begin to develop until the hen starts to incubate them.

Once development starts, the eggs need to be kept warm or the embryo will die. In a clutch you will have eggs that were laid one day ago and eggs laid a week ago, but they will all develop and hatch at the same time because they all began incubation at the same time.

Eggs at the grocery store are stored in a refrigerator to keep them from going bad. How do incubated eggs not go bad during all this time?

Again, the bloom is the key to keeping the eggs safe. Eggs go “bad” when bacteria enter through microscopic pores in the egg shell. If the bloom is not washed off, eggs can safely be stored without refrigeration for at least a month. Once the bloom is washed off, refrigeration is needed to keep eggs fresh. Many backyard farmers keep their farm fresh eggs out on the counter, unwashed.

What does a chick eat inside the egg?

Going from a clump of cells to fully formed fluffy chick in just 21 days takes a lot of energy! Human babies have a placenta to help them get nutrients, and baby chicks have a yolk. The yolk has everything a growing chicken embryo needs. Once hatched, the chick will have enough yolk stored up in it’s system so that it doesn’t need to eat for the first 24 hours while it waits for all of it’s brothers and sisters to finish hatching and mom can take them all out.

How does a chick breathe inside an egg?

Under the shell are two membranes, between those membranes is an air sac to provide the chick with an oxygen source. But as the growing chick breathes in the oxygen, it needs to expel carbon monoxide. That carbon monoxide needs to escape the egg, and the air sac needs to be replenished with fresh oxygen. If you look very closely at an egg shell, you will see it is not a solid surface, but has nearly 7000 tiny pores that allow oxygen in and carbon monoxide out.

Can a chick hear inside the egg?

The tiny pores in the shell can also allow the chick to hear it’s mother as soon as it’s hearing has developed around day 12 of incubation. Chicks have shown that they recognize their mother’s voice at hatching. Towards the end of incubation and during hatching mother hens have been observed clucking quietly to their eggs, singing them lullabies and encouraging them during hatching.

In the last day before hatching, chicks can be heard peeping inside of the egg. If you hold a developing egg cupped between your hands, you can feel the chick moving inside. So even though momma hen isn’t growing her babies inside of her, she can still feel them moving and begins bonding with them.

How does a chick break out of the egg?

The chick has an egg tooth on it’s beak that will fall off during it’s first week of life outside the egg. This egg tooth is an important tool for escaping the egg. A chick will use the egg tooth to break a tiny hole in the shell. It will then begin to turn in a circle inside the egg, breaking tiny holes at it goes to “unzip” the shell. This is a lot of work for a tiny baby! They take a lot of breaks and this process can take them up to 24 hours.

It can be hard to watch them struggle to hatch knowing how much work it is, but this struggle is really important for the chick. A chick pulled out too soon can bleed to death. The struggle also helps strengthen their lungs and legs.