In our last post we talked about the lovely brown eggs our chickens share with us and we raised the question of how and why birds go to the trouble of coloring their eggs. Today, we’ll provide some answers.
A bird egg begins its life in the ovary which deposits the ova, one at a time, into a funnel – called the infundibulum, today’s big word – which swallows it up and sends it down the oviduct, the tube which transports the egg to the outside world. Because the outside world is the egg’s ultimate destination, it is going to need a shell to protect the embryo and it gets it during its passage through the oviduct, a process that usually takes about 24 hours. (That’s a Bar-tailed Godwit egg.)
The shell comes late in the process. First the ovum is dumped into the funnel. The ovum, by the way, is the yolk and it is fertilized in the funnel, a/k/a the infundibulum. (Use a word three times and it’s yours.) As the yolk heads down and out, it passes through a region of the oviduct known as the magnum where the first layer of egg white (albumen) is added. After the magnum comes the isthmus where more layers of egg white are added and the shell begins to form. Before finally reaching the vagina, the cloaca and then the outside world, the egg passes through the last division of the oviduct: the shell gland. Here the last of the egg white is added and the shell forms. Not only is the shell formed here, it gets painted here. The egg spends most of its 24 hours prior to getting laid here, usually around 20 hours. That time varies among species and is dependent on the size of the egg for which a shell must be manufactured and painted.
We should not fall into the idea that the shell is something simple. It isn’t. Made of calcium, mostly, it has to be hard enough to protect the embryo until it is a little bird; ready to punch its way out of the egg; but porous enough that air can get in and out. All the materials, including water, that the embryo will need are in the egg, except for enough oxygen. Nor is the egg big enough to contain all the carbon dioxide the embryo will expel before it cracks the shell. And finally, the shell can’t be so hard that the baby bird can’t fight its way out when it is ready.
What the shell looks like when it enters the outside world depends on the speed and rotation of the egg’s passage through the shell gland. Fast movement leads to streaked eggs; slower movement to spotted eggs. The bird paints the egg in much the same way an artist might paint a canvas, beginning with the background or base color. Bernd Heinrich compared pigmenting an egg to innumerable still brushes painting while the canvas is in constant motion. Something like an ink jet printer, I suppose. The pigmentation is laid down by glands excreting the color as the egg spins down through the shell gland. The spots, streaks and darker colors come next. Because the egg heads down the oviduct with its fat end first, that end of the egg usually gets more color than the thin end.(That’s an egg of the Paradise Riflebird of Australia.)
Now, we’ve reached the thin end of this post. In the next post of this series, we’ll discuss the theories of why birds go to the trouble of coloring their eggs. It is not effort free; the pigments have to be produced, foods must be eaten to help produce the pigments and energy expended. We can assume there is some reason for all that effort, and while it is possible female birds are just artists at heart, most scientists are not willing to give them credit for artistry and look for other reasons.
One more time: infundibulum.