The Color of Eggs, Part III

As you can see from the photo, our Araucanas have begun laying their blue eggs. You may remember from our earlier post that most of our chickens did not begin laying until I explained fried chicken to them. That did not work on the Araucanas and it wasn’t until this week that I figured out why: Araucanas are from Peru and don’t speak English. I don’t speak Quecha but fortunately both the chickens and I could communicate in Spanish. Can you say “fried chicken” in Spanish?

Which brings us to Part III of our series on the color of bird eggs. As we mentioned at the end of our last post in the series, scientists are not inclined to give credit to female avian artistry and look for scientific explanations of why birds go to the trouble of coloring their eggs. They don’t accept our “art hypothesis.”

Their answer is adaptation. The theory goes like this: To survive, bird eggs must protect the small birds inside until they can peck their way out. If egg-eating predators have an easy time finding the eggs, it is much less likely that little birds will live to hatch and then reproduce the species. After all, skunks don’t care whether the egg they eat today means no eggs for their grandchildren. (Or we assume they don’t.)

Like lawyers adducing evidence at trial, the scientists bring forth first, Exhibit A: the Killdeer, which lays eggs in the open. Such eggs would be easy sightings for visual predators unless well camouflaged, as Killdeer eggs are. Coupled with the small similarly marked and colored rocks which Killdeer frequently use to line their nests, the entire apparatus is practically invisible which obviously aids in Killdeer survival. (Killdeer are experts at distraction displays which we discussed briefly in our post about Rikki-tikki-tavi. They are ground nesters.)

Exhibit B consists of white eggs laid by birds which nest in dark holes such as Petrels, Woodpeckers, Kingfishers. White eggs are easier to see in the dark. Exhibit C brings more white eggs, this time of species which leave nests frequently but cover the eggs with grasses or other plants in the vicinity before leaving which hides the eggs from flying eyes.

But there is a hole in this theory and it is the White Leghorn hen, that prolific egg layer we and E.B. White talked about in the first post on egg color. The one who would stop to lay an egg even if she was on the way to a fire. If the purpose of egg color is camouflage to conceal the eggs from predators, who is in more need of that than a domestic chicken whose every egg is stolen from her by two legged mammals? Why does she lay white eggs visible for miles?  I think the scientists have a little way to go to finish their explanation. Given what we are learning about bird brains and how evolved they really are, perhaps scientists should take another look at our art hypothesis.

But I can hear them now, “It isn’t just chickens and birds that hide their eggs when they leave their nests who lay white eggs. Exhibit D contains the eggs of Hawks and Owls and other birds which begin incubating their eggs as soon as the first one hatches. No need to camouflage those because the parent is sitting on them.”

We’ll let you know the verdict when it comes in.

Egg Color, Part II

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.

Egg Color, Part I

We restocked ourselves with chicks this Spring. By late August they still had not produced any eggs so we went out and had a chat with them about fried chicken. The very next day we had our first egg and they have been busy ever since. Except for some Araucanas which produce blue eggs, all the eggs we are getting are brown.

That reminded me of an essay E.B. White wrote in response to the English writer and humorist J.B. Priestley. Priestley had published a piece explaining America to his British readers by decrying America’s preference for white chicken eggs over brown ones. Here is Mr. White:

Why is it, do you suppose, that an Englishman is unhappy until he has explained America? Mr. Priestley finds the key to this country in its preference for white eggs — a discovery, he says, that will move him into the “vast invisible realm where our lives are shaped.” It’s a great idea, but one seldom meets an American who is all tensed up because he has yet to explain England.

Mr. Priestley writes that “the weakness of American civilization. . .is that it is so curiously abstract.” In America, he says, “brown eggs are despised, sold off cheaply, perhaps sometimes thrown away.” Well, now. In New England, where I live and which is part of America, the brown egg, far from being despised, is king. . . “The Americans [writes Mr. Priestley] despise brown eggs because they seen closer to nature. White eggs are much better, especially if they are to be given to precious children, because their very whiteness suggests hygiene and purity.” My goodness. Granting that an Englishman is entitled to his reflective moments, . . . I suspect there is a more plausible explanation for the popularity of the white egg in America. I ascribe the whole business to a busy little female — the White Leghorn hen. She is nervous, she is flighty, she is the greatest egg-machine on two legs, and it just happens that she lays a white egg. She’s never too distracted to do her job. A Leghorn hen, if she were on her way to a fire, would pause long enough to lay an egg. This endears her to the poultrymen of America, who are out to produce the greatest number of eggs for the least money paid out for feed. Result: much of America, apart from New England, is flooded with white eggs.

“The English prefer the brown egg,” writes Mr. Priestley, “because it belongs to the enduring dream of the English, who always hope sooner or later to move into the country.” Here I understand what he is talking about: the brown egg is, indeed, because of its pigmentation, more suggestive of country living — a more “natural” egg, if you wish, although there is no such thing as an unnatural egg. . . .

So, you may be wondering, where does that pigmentation come from and how does it get on those eggs? How and why do birds go to the trouble of coloring their eggs? We’ll attempt an answer, but first I’m off to fry myself an egg: a brown one. Although completely irrational, we agree that a brown egg seems closer to nature.