The chick first pierces the membrane of the air chamber to breathe, then cracks the shell using a small horny diamond located on its beak, turning on itself inside the egg to create a crown-shaped cut over several hours, without any external help.
The mechanism of hatching
Around the 19th day of incubation, the embryo has consumed a large part of the albumen and occupies almost the entire volume of the egg. It orients its head under its right wing and pushes its beak through the internal membrane of the air chamber, housed at the large end of the egg. This first gesture allows it to breathe the air trapped in this pocket even before piercing the shell: the lungs then begin to function in parallel with the still active Allantoic respiration.
The chick then uses the diamond, a small pointed limestone growth present at the end of the beak, to create a first crack called "staring". Thanks to the spading muscle of the neck (hypertrophied complexus muscle at the end of development), he gives repeated headbutts which rotate his body a few degrees with each strike. This rotating movement gradually draws a line of fractures in a circle, or crown, around the large end of the egg. This phase, called digging, generally lasts from several hours to a whole day depending on the individual.
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Once the crown is complete, the chick pushes with its legs and its back to tilt the shell cap and emerge. The entire operation, from the first crack to complete exit, often takes 12 to 24 hours.
Why you should never help
The diamond and the digging muscle disappear naturally in the days following hatching, once their role is finished. Intervening by breaking the shell in place of the chick is strongly discouraged: this can cause hemorrhage if the blood vessels of the Allantoic membrane have not finished resorbing, and disrupts the physiological time necessary for complete closure of the umbilicus. In the event of a chick visibly stuck beyond the usual time limit, the advice of a veterinarian or an incubation professional is recommended rather than manual intervention.
