A pip hole is a small crack or hole which is made in an egg by a chick as it prepares to hatch. The appearance of pip holes is generally viewed as a sign that chicks are on the way, as the chicks generally fully emerge within a day or so of creating the pip hole, depending on the species. The process of breaking out of the shell is known as pipping.
Animals which hatch from eggs, such as birds and reptiles, actually have a specially developed anatomical structure to help them break out of their eggs. This structure is known as an “egg tooth,” and it takes the form of a protuberance on the beak, snout, or forehead. Once the animal hatches out, the egg tooth dries up and falls off within a few weeks.
Watching eggs hatch can be quite interesting. At first, the egg will start to shake, as the creature inside moves around and uses its egg tooth to crack the shell. Then, the pip hole will appear, and the animal will slowly enlarge the hole until it can get out of the egg. Chicks often use their legs and feet to thrust their way out of their eggs once they have created a pip hole. This whole process can take an extended period of time, as it requires a lot of strength from the young animal.
After hatching, depending on the species, the animal often takes a break to recover its strength. In the case of animals which are raised by their parents, the parents will bring back food, while others are forced to forage within hours if they want to survive. Because animals tend to be weak after pipping, they are especially vulnerable to predators.
When people raise birds, they often look forward immensely to the hatching, both because it is interesting and because it signals the arrival of the next generation. In some cases, people may also help the chicks get out of their shells, by gently lifting away pieces of the shell after the chicks have made their pip holes. However, caution is advised when doing this, because it is easy to hurt the fragile young birds.
Because a pip hole looks sort of like a peep hole, through which the young animal appears to peer, some people use the term “pip hole” to more generally describe any sort of small hole in an object or structure. For chicks, the view out the pip hole is the first glance of the world, but usually the young animal is so focused on getting out that it doesn't really notice its surroundings.