Birds are homogeneous when it comes to egg laying, that is, they all lay eggs, differing from mammals which birth their young alive, with two exceptions (spiny anteater and platypus – the monotremes) and reptiles, mostly egg layers, but with a number of live-bearing species as well (sea snakes and boas, for example.) Live-bearers are termed “viviparous” while egg-layers are “oviparous”. Let’s stick with birds, all oviparous, for this discussion.
A fully developed chick ready to hatch needs to break out of its egg, the home which nourished and protected the developing bird for the past 10 days to two months. The shell is mainly calcium carbonate, crystals of which are deposited on a gel-like membrane. (If you put a chicken egg in a vinegar solution for a few hours, the calcium carbonate dissolves, leaving the egg in a rubbery, flexible membrane.) But the shell is actually very sturdy. There’s the cool trick of trying to crack a chicken egg in your hand – try it, it’s not easy.
When ready to hatch, the chick faces the task of escaping this enclosure, which means breaking the shell. You might imagine that the bird starts pecking at the egg like a woodpecker pecks at a tree trunk. But you would imagine wrong. The beak of a young bird is not strong enough to penetrate the shell. So evolution has provided a neat solution to this dilemma: the egg tooth. Birds, reptiles, and monotremes all have one.
At some point in late development an egg tooth starts to develop. An egg tooth is a tiny horny projection at the forefront of the upper mandible – not at the beak tip- of an about to hatch chick. As hatching time approaches the egg tooth becomes progressively sharp and hard. Approximately three days before the chick hatches, it uses the egg tooth to break through the inner membrane at the blunt end of the egg, allowing it access to more oxygen which it will need for energy to break out of the shell. And there is an important coadaptation working in conjunction with egg tooth – the hatching muscle.
The hatching muscle forms at the back of the neck, developing along with the egg tooth, allowing the chick to move its head upward so that the egg tooth contacts the shell with enough force to crack the egg, a process called pipping. A few exceptions are the Kiwi and brush turkey (megapodes) whose young are big and strong enough at hatching to kick open the egg with their feet. Woodpeckers, curiously, have two egg teeth, one on the top bill and one on the bottom bill. One research article speculates that the egg teeth in woodpeckers, cavity nesters, increases the visibility of the gape of the young bird displaying to the parent. Both the egg tooth and hatching muscle fall off or are reabsorbed within a week or so after hatching. Reptiles, monotremes, and birds all have egg teeth and hatching muscles, part of the evidence showing the strong evolutionary connection between these groups.