How Chickens Get Oxygen Inside the Egg
When you look at an egg, it seems completely sealed. No tubes, no umbilical cord, no visible holes – yet a chick grows and develops inside for 21 days without suffocating. So how does it get enough oxygen, and where does all the carbon dioxide go?
In this article we’ll take a closer look at the biology inside the egg, why shell pores and the air cell are so important, and what this means for successful incubation in your incubator at home.
Watch: A Brilliant Visual Explanation
The video below from NPR’s Skunk Bear team gives an excellent animated tour of how bird embryos breathe inside the egg. It’s well worth watching before (or after) you read the breakdown.
Video credit: NPR Skunk Bear. Shared for educational purposes. All rights remain with the original creator.
The Egg Is a Life-Support System
A fertile egg is more than shell, white and yolk. It’s a complete life-support system designed to protect the embryo and supply everything it needs until hatch:
- Shell: A hard, calcium-rich outer layer that protects the embryo and also allows gas and water vapour to pass through microscopic pores.
- Shell membranes: Two thin membranes just inside the shell. When the egg cools after being laid, these separate at the large end and create the air cell.
- Air cell: A pocket of air at the large end of the egg. This is the chick’s emergency oxygen supply just before hatch.
- Yolk and albumen (white): Food and water reserves that fuel growth.
- Chorioallantoic membrane: A specialised membrane full of blood vessels that acts like a temporary “lung” pressed against the inside of the shell.
From the outside the shell looks smooth and solid, but in reality it contains thousands of microscopic pores. These pores are the key to oxygen getting in and carbon dioxide getting out.
Shell Pores: Invisible Doorways for Gas Exchange
Each chicken eggshell contains many tiny pores that run from the outer surface to the shell membranes. Through these pores:
- Fresh oxygen from the surrounding air diffuses in towards the membranes.
- Carbon dioxide (a waste product of metabolism) diffuses out of the egg.
Diffusion simply means gas molecules naturally moving from an area of higher concentration to an area of lower concentration. As the growing embryo uses oxygen and produces carbon dioxide, a gradient is created that constantly pulls fresh oxygen in and pushes carbon dioxide out through the pores.
This process is passive – there is no “pumping” mechanism. It depends entirely on:
- Healthy shell structure and pore number
- Clean, unblocked shell surfaces
- Good airflow and oxygen levels in the incubator or under the hen
The Temporary “Lung” Inside the Egg
Very early in development, the chick embryo grows a network of blood vessels that spread out over the inside of the shell membranes. This structure is called the chorioallantoic membrane.
Functionally, this membrane behaves like a lung:
- Deoxygenated blood from the embryo flows out to the membrane.
- At the membrane, oxygen diffusing in through the pores is absorbed into the blood.
- Carbon dioxide diffusing out of the blood passes through the pores to the outside air.
Because this membrane is pressed right up against the shell, it can make use of the entire shell surface area for gas exchange. For almost the whole incubation period this is the chick’s main breathing system.
The Air Cell: First Breaths and the Internal Pip
As the egg cools after laying, the contents contract slightly and the shell membranes pull apart at the large end, forming the air cell. This pocket of air becomes crucial in the final days.
Near day 19–20 for chickens, the chick turns so its beak is positioned towards the large end of the egg. When it is ready, it performs an internal pip:
- The chick pecks through the inner shell membrane and opens into the air cell.
- For the first time, it starts to breathe air directly with its own lungs – still inside the shell.
- The chorioallantoic membrane continues working for a short time while the lungs take over.
Over several hours, the chick switches gradually from membrane-based gas exchange to lung breathing. Once it is stable and strong enough, it performs the external pip – the first small crack you see at the shell surface.
Why Incubator Conditions Matter for Oxygen
Because everything depends on diffusion through the shell, incubator conditions can strongly influence gas exchange. Key factors include:
Ventilation and Fresh Air
- Ventilation holes must remain open so the air around the eggs contains enough oxygen and doesn’t build up excess carbon dioxide.
- Blocking vents, running an incubator in a very confined space, or over-packing with eggs can reduce oxygen availability.
Humidity and Air Cell Size
As the embryo grows, it uses water from the egg and water vapour leaves through the pores. This controlled water loss:
- Allows the air cell to grow to the correct size.
- Ensures there is enough oxygen in that pocket for the chick’s first breaths at internal pip.
If humidity is too high for most of the incubation:
- The egg loses too little water.
- The air cell may be too small.
- The chick can struggle to breathe properly after internal pip.
If humidity is too low:
- Excess water loss can dry the membranes.
- The chick risks becoming “shrink wrapped” – stuck tight in dry membranes that restrict movement and breathing.
Temperature and Hatch Timing
Temperature affects the chick’s metabolic rate and therefore its oxygen demand:
- If eggs consistently hatch early, the average temperature has probably been a little too warm.
- If eggs consistently hatch late, the average temperature has probably been a little too cool.
After a couple of hatches, you can fine-tune your incubator by very small adjustments (for example, 0.1–0.3 °C) based on whether your chicks tend to arrive early or late. This is a simple, practical way to “calibrate” your setup to match nature’s thermometer.
When Things Go Wrong Near Hatch
Because the chick’s oxygen needs are highest in the last few days, problems with ventilation, humidity, or shell condition are most obvious right at the end. Signs of trouble can include:
- Fully formed chicks that die just before or during pipping.
- Chicks that externally pip but then stop progressing for many hours.
- Weak, gasping chicks that struggle to stand after hatching.
Common contributing factors are:
- Inadequate ventilation – vents blocked, incubator placed in a poorly ventilated cupboard, or too many eggs crammed in.
- Incorrect humidity over the whole incubation – air cells too large or too small.
- Opening the incubator too often at hatch – causing rapid humidity drops and membrane drying.
Good preparation, reliable equipment and steady settings are usually enough to avoid most of these issues.
Practical Tips for Better Oxygen Supply
- Do not block or tape over the incubator’s ventilation holes.
- Place the incubator in a room with reasonable fresh air flow, not sealed in a small cupboard.
- Monitor humidity and aim for appropriate weight loss or air cell growth for your egg type.
- Avoid opening the incubator once you see external pips – every opening drops humidity and can dry the membranes.
- Use a quality, well-positioned thermometer or sensor and resist the urge to keep changing settings without a clear reason.
Remember: the chick and the egg are doing the complicated work. Our job is to provide stable, sensible conditions so this amazing natural system can do what it was designed to do.
Learn More About Hatching and Incubation
If you found this explanation helpful, you may also like our other guides on incubation and troubleshooting. Together they’ll give you a solid understanding of what’s happening inside the egg – and how to turn that knowledge into stronger, healthier hatches.