Have you ever wondered why eggs are shaped the way they are? Or why egg shape varies with the species? Most eggs have one tapered and one wider end to ensure they roll only in a circular pattern. This ensures that the eggs do not roll out of the nest when they are turned by the parents. Some species lay eggs with a more pronounced small end, which make them roll in a tight circle. For example, seabirds like murres nest strictly on rocky cliffs and use no nest material at all. The elongated shape of their egg makes it less likely to roll off the cliff edge. Birds that make deep cup-shaped nests typically have rounder eggs, because there is less risk that the eggs will roll out of the nest when turned. Nature’s exquisite engineering continues inside the egg, too!This alala egg shows that “egg-shaped” is a broad term for a range of shapes.
The egg itself is a self-contained home, supplying all the nutrients and safety for the growing chick inside. The duration of incubation varies, but all chicks will grow until there is so little space inside that it is difficult to move. When it is time to hatch, the chick has to get into the proper position, with the head under the right wing and the beak pointed upwards toward the larger end of the egg (being in the wrong position can be fatal). The large end of the egg has an air-filled space called the air cell. The chick must use its beak to pierce the air cell membrane so it can start breathing the air inside. As CO2 builds up in the air cell, it triggers the yolk sac to retract into the chick’s abdominal cavity, getting the bird ready for life outside the egg. The next step is to break out of the shell.
The beak has a hard, sharp, triangular shaped structure, called an egg tooth, on the top of the beak that assists in breaking through the eggshell. The chick uses its legs to rotate as it pecks through the shell until a hole is large enough to break out of completely. Once outside the shell, the chick can rely on its yolk sac for energy and nutrients until it is getting enough food from its parents, or is feeding on its own.
With such a complicated process there are bound to be occasional problems with completing the incubation and hatching process. But when these problems arise, we can learn from them and provide helpful feedback for our animal care staff, enabling them to make management adjustments that will maximize the reproductive success of the many amazing species of birds we have in our care.
April Gorrow is a Senior Pathology Technician at the San Diego Zoo Institute for Conservation Research.