Why Do Fully Developed Chicks Die Right at the End?

Few things are more disappointing than opening an incubator to find a fully developed chick that never managed to hatch.

It's probably the most common question I'm asked:

"Why did it die right at the end?"

The answer is often surprising.

The place where a chick dies is frequently not where the problem began.

In many cases, the embryo's fate was determined days earlier during incubation. The final stages of hatching simply expose weaknesses that have been developing throughout the incubation period.

The Two Physiological Choke-Points

I think of the final stages of hatching as two critical physiological choke-points.

Every chick must successfully pass through both if it is to hatch.

Choke-Point One – Internal Pip

As the embryo grows, its oxygen requirement increases dramatically.

Until this stage, all of its oxygen is supplied through the shell by diffusion into the blood vessels lining the chorioallantoic membrane.

Eventually, however, that system reaches its limit. Oxygen can no longer enter the egg quickly enough to satisfy the embryo's increasing metabolic demand, while carbon dioxide begins to accumulate.

This creates a powerful physiological stimulus.

The embryo responds with vigorous convulsive movements and repeated head thrusts until the egg tooth ruptures the inner shell membrane and enters the air cell. This event is known as the internal pip.

For the first time, the chick begins breathing air with its lungs.

Many embryos never successfully complete this transition.

Choke-Point Two – External Pip

The air contained within the air cell is only a temporary reserve.

Once lung breathing begins, oxygen demand continues to rise rapidly. The limited volume of air inside the air cell is soon exhausted, carbon dioxide again accumulates, and the chick faces a second respiratory crisis.

Once again, vigorous head movements drive the egg tooth through the shell, producing the external pip and providing continuous access to fresh air.

Only then can normal breathing continue.

Again, some embryos fail to complete this second critical transition.

Why They Fail

The important point is this:

Failure at either choke-point does not necessarily tell you what caused the failure.

The underlying cause may have occurred much earlier and can include:

Incorrect incubation temperatures.
Poor humidity management and incorrect egg weight loss.
Nutritional deficiencies in the breeding flock.
Malposition of the embryo.
Genetic abnormalities.
Disease.
Physical abnormalities.
Or simply an accident of nature.

The chick dies at one of these choke-points because that is when the demands placed upon its body are greatest—not because that is necessarily when the original problem occurred.

The Engineering Perspective

As an engineer, I see these stages as the biological equivalent of a stress test.

A structure may contain a hidden weakness for years without failing. It is only when maximum load is applied that the weakness becomes obvious.

The same principle applies to embryo development.

The internal and external pips are the points of maximum physiological demand. They reveal problems that may have existed throughout incubation but remained hidden until the embryo was asked to perform its most difficult tasks.

The Takeaway

When investigating late dead-in-shell embryos, don't focus only on the moment of death.

Instead, ask what happened during the previous 21 days that left the chick unable to negotiate one of the two critical physiological choke-points of hatching.

That change in thinking is often the difference between simply explaining a poor hatch and preventing the next one.