Development of a chick in the egg is a fascinating journey. Raptor chicks, like most species of birds, must break out of the raptor egg with a process known as pipping (fig 1). A pipped egg is when the raptor chick is beginning to hatch, breaking through the egg membrane and shell. To facilitate this part of hatching, the chick is equipped with an egg tooth (fig 2).
Development of the egg tooth begins approximately on the seventh day of incubation. A small pointed keratinized process, the egg tooth, will form on the rostral part of the upper bill. The chick, to break through the layers of the eggshell to hatch, uses the egg tooth. The tooth along with strong upward and backward thrusts of the head achieves the actual pipping. This movement is made possible by the large, strong pipping muscle on the back of the neck. Once the chick is hatched, the egg tooth will not continue to grow or develop and will fall off by two weeks of age.
The pipping muscle or hatching muscle is a paired structure located on the dorsal neck of the bird. As the chick prepares to hatch this muscle will swell with lymph a few days before hatching and reaches its peak size at the time of pipping. Following hatching, this muscle will lose its fluid and assume a normal appearance. In the adult bird, this muscle (Musculus complexusor), functions to raise or elevate the head.
Not all birds have an egg tooth. Megapodes are large-footed, ground dwelling birds in the East Indies, South Pacific and Australia, and lack an egg tooth. These chicks simply kick their way out of the egg. The chick of the parasitic honeyguide bird (lays eggs in a host species nest) has a pair of sharp hooks on the upper and lower beak. The chick will use this to break through the egg and then kill the nestling of the host species. Some reptiles will also have an egg tooth similar to birds. However with snakes and lizards, this structure is a true tooth containing cement and dentine.