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The process of hatching is a complex and fascinating biological phenomenon, which has been studied extensively by avian biologists. It involves the development of the embryo within an eggshell, followed by its emergence in a newly-hatched form.

This article will explore how chicks get out of the eggshell, highlighting the physical and physiological processes involved.

The growth and development of embryos inside an egg requires energy and nutrients that are provided through yolk reserves stored within the shell. During this period, several changes occur both to the chick itself and to the structure of the eggshell.

These changes enable successful hatching from eggs, allowing for continued life cycle progression of many bird species.

Chick hatching

Eggshell Structure & Composition

The eggshell is a complex structure that provides protection and support for the developing embryo. It consists of calcium carbonate, proteins, lipids, and other organic components which together form a strong but porous membrane.

Researchers have long hypothesized that yolk sac development affects the rate at which an eggshell matures; however, this has yet to be proven conclusively.

Studies show that avian species with larger yolk sacs tend to lay eggs with thicker shells than those with smaller yolk sacs. This may suggest a relationship between the two variables; however, further research is needed in order to determine whether there is indeed a correlation between the size of the yolk sac and the thickness of an eggshell.

In any case, it is clear that both factors play important roles in proper eggshell maturation and overall chick hatching success.

Eggshell Hardening & Softening

The eggshell is an essential part of the hatching process, as it protects and shelters the developing chick. The shell must be strong enough to protect the embryo from external factors such as bacteria, fungi, predators and dehydration without constricting its growth or development. To meet this challenge, two main processes are used: shell hardening and softening.

Shell hardening occurs during late embryonic development when calcium carbonate accumulates on both sides of the internal membrane creating a protective barrier. This increases pressure within the inner chamber which helps reinforce the strength of the shell by strengthening its structure.

During this same time period, glycoproteins accumulate along the surface layers forming a waterproof layer known as cuticle that prevents water loss from inside the eggshell.

To facilitate hatching, several days before emergence, eggshell softening takes place due to increased levels of hormones such as estrogen and progesterone produced in response to environmental cues like light stimuli or temperature shifts. These hormones induce molecular changes in eggshell proteins causing them to weaken allowing for easier penetration of chorionic structures so they can break through the outermost layer of their enclosure thus completing the hatching process.

In addition to these two mechanisms there are other important elements related to successful incubation including:

  1. Temperature regulation- maintaining optimal temperatures throughout incubation will ensure proper embryo development
  2. Humidity control- adequate humidity levels help reduce water evaporation while avoiding bacterial growth
  3. Turning eggs- periodic turning ensures uniform distribution of nutrients throughout embryogenesis
  4. Shell reinforcement- supplementing nests with additional material may further strengthen shells against predation

Yolk Reserves & Growth Of The Embryo

The eggshell is a fascinating structure that serves many important functions. It protects the embryo from predators and environmental hazards, while also providing insulation to regulate temperature. Additionally, it provides nutrition in the form of yolk reserves for the rapid growth of the embryonic development.

These yolk reserves are stored within the albumen and chalazae of the egg. Albumen consists mostly of protein and water, acting as an energy source to fuel early stages of embryogenesis. The chalazae are two twisted strands composed mainly of lipoprotein which anchor the yolk at opposite ends of the egg so that it does not move around during incubation. Together these components provide nourishment for one of nature’s most impressive developments – avian embryogenesis.

EggshellProtects embryo from predators & hazards
Insulates against extreme temperatures
AlbumenProtein & water energy source
ChalazaLipoprotein anchors yolk to either end
of egg preventing movement

As these elements combine together they create an ideal environment for incredible feats such as hatching a fully formed chick without any assistance or intervention by humans. This remarkable process requires careful study and research by avian biologists to ensure proper nutrition is supplied throughout each stage of development leading up to emergence.

The Role Of Temperature And Humidity

The hatching process of a chick requires an interaction between the egg and its environment. Temperature, in particular, is essential for chicks to hatch from their eggs. Through incubation, the temperature inside the egg can be regulated to optimal levels by controlling humidity as well.

The ideal hatching temperature has been estimated at around 37°C (98.6°F). Higher temperatures may cause developmental malformations or even death in some species. For instance, studies on domestic chickens have shown that high relative humidity during incubation can lead to improved embryonic development and decreased mortality rates.

This occurs because increased moisture leads to better gas exchange in the eggshell membrane which allows oxygen supply within the shell and removal of carbon dioxide from it. Consequently, adjusting the environmental conditions such as temperature and humidity control are key factors influencing successful hatching of chicks.

Cervical Flexion & Piping

Incubation is the process by which a fertilized egg develops into an embryo. During this period, embryonic growth takes place through a series of cellular divisions and organogenesis until hatching occurs.

In order to help facilitate hatching, chicks undergo cervical flexion or pipping as it is often referred to in avian biology. Cervical flexion involves the chick’s head being bent down towards its legs and chest while pushing against the inside of the eggshell with its beak; a behavior that can be observed during egg candling examinations.

This action allows for air exchange between the outside environment and within the airspace found in most eggs. As oxygen enters, carbon dioxide and water vapors are expelled from the egg, thus reducing humidity levels near where piped holes have been created.

The reduction in internal pressure caused by these activities also reduces adhesion forces acting on the inner surface of the shell leading to an easier breaking process when pecking at weakened areas around pipped holes become more frequent.


The egg tooth is a small, temporary projection found on the beaks of many bird species, including chickens, ducks, and songbirds. It plays a crucial role during the hatching process and has several important functions:

  1. Pipping the Eggshell: The primary purpose of the egg tooth is to assist the bird in breaking through the eggshell during hatching. The egg tooth is used to create a small hole or “pip” in the shell, allowing the bird to exert pressure and eventually emerge from the egg.
  2. Absorption of Calcium: The tip of the egg tooth is composed of a specialized keratinized structure that contains a high concentration of calcium. This calcium helps in the absorption of nutrients from the eggshell, providing additional nourishment to the developing chick as it hatches.
  3. Protection from Injury: The egg tooth is covered by a protective sheath, preventing the chick from accidentally injuring itself or damaging delicate tissues inside the egg during the pipping process.
  4. Temporary Structure: Once the bird has successfully hatched and emerged from the egg, the egg tooth usually falls off or wears down shortly after. It is not a permanent feature and is only present during the hatching period.

Overall, the egg tooth is a specialized adaptation in birds that facilitates the hatching process, aiding in the successful emergence of the chick from the eggshell. It allows them to break through the shell and start their life outside the protective confines of the egg.

External Assistance & Assisted Hatching

Cervical flexion and piping are two of the primary methods by which chicks emerge from eggshells. However, there are occasions when external assistance is required to ensure successful hatching.

This can involve opening the eggshell or providing increased humidity through misting. In some cases, assisted hatching may be necessary for a chick to break free from its shell.

This process involves creating an artificial pore with a surgical blade that allows the embryo to breath prior to breaking out on its own. The pores created in this manner must be large enough to allow gas exchange but not too big so as to damage the inner membrane of the shell.

Calcium absorption is another factor that can affect embryonic development and ultimately success at hatching. During incubation, calcium ions pass through permeable membranes into the amniotic fluid surrounding the developing embryo allowing it to utilize them in muscle contraction during pip movements and cervical flexion attempts.

If adequate levels of calcium are not present, then these processes will be impaired resulting in difficulty hatching without intervention. Therefore, maintaining appropriate levels of calcium throughout incubation is crucial for successful unassisted hatchings.

Chick hatching
Newborn yellow chicken hatching from egg


The development of an avian embryo is a complex and fascinating process. As the chick develops its muscles, organs and skeleton within the confines of the eggshell, it must eventually break through this protective barrier in order to hatch.

The temperature and humidity surrounding the egg play key roles in regulating when hatching will take place, while the reserves in the yolk provide critical nutrition for growth.

In addition, both cervical flexion and piping are important mechanisms used by chicks to weaken their shells in preparation for hatching.

Finally, certain species may require assistance from external sources or assisted hatching methods as they work towards breaking free from their eggs.

Avian embryology continues to be a captivating field of study due to its complexity and importance for bird survival in nature.