An eggshell does a good job of protecting a developing chick, stopping anything from getting in, but it also, eventually, has to let the chick out.

So how exactly, do eggshells resist damage yet are weak enough to be broken by the hatching chick, which can hardly be described as the strongest creature in the world.

Researchers in Canada would now appear to have solved this puzzle, and the answer would seem to lie in the changing structure of the shell as it is broken down by the developing chick drawing from it the resources that it needs.

A team from the McGill University’s Faculty of Dentistry, with Faculty of Engineering and other colleagues, has looked at the molecular nanostructure and mechanical properties of eggshell interiors using new sample preparation techniques, and have published their work in Science Advances.

As the chick develops, it will need calcium to form bones, and this calcium comes from the shell. During incubation, the inner portion of the shell partially dissolves to aid bone formation, and calcium is drawn to the chick and the shell weakens, making it easier for the hatching chick to break.

A factor determining shell strength is the presence of a nanostored mineral associated with osteopontin – a protein found in both eggshells and bones.

The three layers of an eggshell all have fine nanostructures, varying in size depending on location. Within the outer layer, these nanostructures are smaller and closely arranged, which results in strength, while in the inner layers they are progressively larger.

As the inner layers are partially dissolved to aid in the formation of the chick’s skeleton, these structures become smaller, and so weaker.

After 15 days of incubation, the researchers found that the outer layer of the shell remained relatively unchanged, despite the inner layers becoming weaker, meaning that while the chick remained protected, its task of hatching had becoming ever-easier.

In addition to shedding light on how chicks actually manage to hatch, the researchers hope that the discovery may help in selecting layers that produce stronger eggshells, resulting in safer eggs, and that their work may help in the development of man-made materials.