Archeologists have found fossils that indicate that birds exist on earth already for approximately 150 million years. They initially looked more like small dinosaurs than like the creatures we know nowadays, but since about 66 million years they look quite equal to the birds that we know, although at that time we would perhaps not call it a chicken yet.

But it means that process like incubation date back a long time, and although birds might not be very intelligent, they have millions and millions of years of experience. This probably means that over the years they have figured out how to do certain processes like for example incubation, and what is important in that process and what is not.

If we look at modern incubation, we build big machines that can hold 100.000 eggs or more, and we regulate the temperature, humidity and ventilation in a very precise way. Especially air temperature is a factor that is of utmost importance, and hatchery managers can spend hours on discussing if the temperature of the air at a certain moment in time and for a certain batch of eggs should be 0.1^oC higher or lower. And they do have a good reason for it, as a small shift in temperature can have quite a significant influence on hatchability and/or chick quality.

But this raises the question how mother hen is actually controlling it. Anybody that has seen a chicken build a nest and start incubation, knows that she doesn’t seem to be very precise in controlling the site of the nest. It can be somewhere in a hay stack, or on the floor, in a chicken coup but also in an old pot or pan. She lines the inside of her new nest with some materials that she finds in the neighborhood, produces the eggs and starts doing her thing for the next 3 weeks. And usually with good results, as can be expected after 66 million years of experience.

But how does she do it? Because if you look at the conditions in and around the nest, she herself might have a constant body temperature but it is questionable if the environment in the nest results in a temperature as constant as the less than 0.1^oC fluctuation that we like to create in our machines.

The secret is that she is not controlling the temperature that precise at all, she leaves it to the embryo. At a body temperature of 41^oC, the brood path of the hen has a temperature of 40^oC, but the bottom of the nest can have a whole range of temperatures, 20^oC, or 25^oC, or whatever. When the hen starts incubating, she warms up the top of the egg with her body through contact temperature. The bottom of the egg touches the nest and will stay relatively cold, creating a gradient of temperature through the egg. The embryo is approximately 7 mm under the shell, and is warmed very rapidly by the mother. After a couple of days, the embryo forms the blood vessels over the yolk, through the egg towards the bottom. And then the big secret starts. When the embryo is feeling that it is getting too warm, it sends more blood to the bottom of the egg, and when it is too cold it keeps more blood at the top of the egg. So the embryo uses the temperature gradient between the breast of the mother and the bottom of the nest to regulate its own temperature.

Researchers from the university of Berlin have been able to measure this blood flow, and noticed that the embryo already starts redirecting the blood when its internal temperature changes 0.1-0.2^oC. So, a difference in 0.2^oC is enough for the embryo to recognize that the temperature is not optimal anymore and that it should react to this by changing its blood flow.

Now what is different between natural incubation in a nest and artificial incubation in a big box with a specific temperature, called an incubator? In an artificial incubator there is no gradient in temperature for the egg, as the air with a specific temperature is surrounding the egg from all sides. So, when the embryo is getting too warm, it can direct the blood in any direction it likes, but it will not change in temperature. And that is the reason why hatchery managers are so obsessed with small changes in air temperature, because they have to do that job for the embryo.

Artificial incubation is a very precise process that requires maximum control. The reason that we need to be so precise is that we do not allow the embryo to create her own optimum. We do not have to do incubation in the same way as mother hen does it, but as the embryo after 66 million years still expect mother hen to sit on top of her egg, we better give the embryo the conditions that it otherwise would create for itself.