Whereas most mammals maintain a protected internal environment for the growing fetus, birds use an external protective covering consisting of calcium called a “shell.” This protective shell provides an isolated environment to allow the developing chick to thrive but once the chick is ready to survive in the outside world, this secure vault needs to be opened. The chick needs to break open the shell without help, in its own time frame and only the chick’s internal clock knows when it is time to emerge. Therefore, the chick needs a device to break through this hard, protective covering. The beak and claws of the chick are not yet strong enough or sharp enough to break the shell. The “egg tooth” is the unusual structure, only found in emerging chicks and lost soon after hatching that penetrates the hard shell that was once the protector of the embryo.
The egg tooth is an essential component to the hatching process in almost all species of birds. This specialized structure develops for the sole purpose of “pipping” (initial breaking of the shell) and cutting the chick out of the egg. The egg tooth firsts breaks through the internal shell membrane. This happens at least a few hours before hatching. At this point, the chick starts breathing by using the internal air cell in the egg. In some species, the chicks all might start chirping at around the same time as a method of communication. This may be an effort to coordinate the timing of hatching in the nest. In another few hours or more, the egg tooth is used to break the external shell. In parrots, the range of time between internal and external pipping can be from 3 to 72 hours. From this point on, it may take up to 24 hours for the chick to entirely break through the shell and emerge into the world.
The egg tooth is sharp as it needs to break the shell and cut the chick out of this strong, protective, calcified shell. The egg tooth develops from the edge of the material of the top beak or rhinotheca. The chick first uses the egg tooth to cause a crack in the eggshell. Once this crack has appeared the chick back-stretches the neck and uses the sharp, long edge of the egg tooth to cut through more of the shell. The egg tooth, in most species, falls away or is worn down in the first 2 to 4 weeks of life as the beak material continues to grow and the egg tooth is not replaced. The breaking of the shell by the egg tooth is aided by the enlarged “pipping muscle” in the neck region. This helps by cushioning the head and bracing the neck to allow the strong movements needed by the body of the chick to push the egg tooth through the shell.
The process from the internal pipping to breaking through the external shell and emerging into the outside world is gradual in most birds. This allows the physiology of the chick to gradually, without shocking the system, begin to adjust to breathing air and the new found unlimited mobility of its limbs. Both the respiratory and circulatory systems must adjust to this new atmosphere and success depends on a gradual changeover rather than an abrupt ending of the protected shell environment. The yolk sac, which will no longer be necessary once the chick escapes the egg, starts to retract and is absorbed by the newly emerged hatchling. In weaker chicks, this whole process will take longer and there is always the temptation for people to “help” the process along. This is usually a mistake as the respiratory and circulatory systems are not yet developed enough to support the chick in the outside world and the yolk sac might still be largely visible. More dangerously, the vessels attached to the yolk sac may still be functioning and, if disrupted prematurely, can cause a fatal bleed out.
There is some variation on the egg tooth in the avian world. As usual, we find the kiwi does this process differently. The kiwi is almost always the exception that proves the rule in the bird world. Kiwis do not have an egg tooth but instead will use their legs along with their beak to break through the relatively thinner eggshell of this species. Many species of woodpecker (including the flicker) have two egg teeth, one each on the tip of the upper and lower beak. In fact, some speculate there is more of a purpose to the egg tooth in woodpeckers and other species than just to pip the shell for hatching. The egg teeth are visible to parent woodpeckers in very low light allowing the parent to more easily find the oral cavity of the nestlings, improving feeding efficiency to the chicks. Flickers and some burrow-nesting seabirds retain their egg teeth until fledging, which is useful when trying to feed nestlings in the dark.