We all know what a chicken egg looks like—hard shell, gooey clear stuff inside that turns white when you cook it, yellow yolk in the middle. You may have noticed that twisted “umbilical cord” and maybe you fished it out before frying your breakfast. If you break an egg into a bowl, you’ll find that the white (the albumen) has a thicker part around the yolk, and a thinner part further out. And if you’ve ever peeled a hard cooked egg, you might remember two layers of translucent membrane just inside the shell—removing them as you go makes it easier to get the shell off.
But have you ever really looked at an egg? Wondered what all the parts do? An egg is actually an amazingly sophisticated way of protecting and providing for a developing bird embryo.
Let’s start with the yolk. A lot of people believe that the yolk develops into the baby chick. Not true. If you look very closely, you’ll see an amorphous white spot on the surface—that’s the germinal disc, or female ovum. If the egg has been fertilized, that ovum fuses with a sperm and begins to divide. By the time the egg is laid, the white area has a distinctly circular shape with a defined edge and is now called a blastoderm, a group of cells forming the beginnings of an embryo. That is the baby chick.
Mammals have an umbilical cord linked to an ongoing food supply, but bird embryos are on their own, isolated in a self-contained egg. The yolk is there to provide nourishment for the developing chick. It actually contains more protein than the albumen, along with fat, iron, vitamins A and D, thiamine, riboflavin, calcium, and lecithin.
If the yolk provides the food, what does the white do? It’s hard to tell from an egg we’re eating, but there are actually four concentric layers of egg white with an alternating thick and thin consistencies. These layers protect the developing chick and provide additional proteins—over 40 kinds—plus water. You can see two layers in the photo above—a thicker one surrounding the yolk and a watery layer spreading out around the edges.
Now what about that twisty cord that gets in the way when we’re cooking? You can easily see it in the photo. Its official name is the chalaza, and it’s there to keep the yolk, and therefore the embryo, centered in the egg.
Eggs also contain an air space inside them. Older eggs have larger spaces, as the liquids evaporate through the shell. That air space aids in gas exchange, allowing the embryo to obtain oxygen and dispose of carbon dioxide long before it takes its first breath.
Outside, on the inner surface of the shell, are the two membranes I mentioned earlier. Those act much like our skin, keeping bacteria out (crucial in a soiled nest!), while allowing for gas exchange with the environment.
Finally, we have the eggshell, a masterpiece of calcium carbonate crystals in a protein matrix. Both air and water pass through thousands of microscopic pores in the shell. When a hen lays an egg, she applies a wet blanket around the shell, the bloom, that dries to an invisible barrier against bacteria. If you want your eggs to last in the refrigerator, don’t wash that bloom off until you’re ready to use them.
Eggshells come in various colors. Birds that nest in dark cavities tend to have white eggs, while those with exposed nests are more camouflaged, disguising their precious contents from predators, such as with these Killdeer eggs. Pigments in egg shells also serve to strengthen the shell. Eggs laid later in a clutch tend to be more colorful, with the pigments increasing as the female bird’s calcium levels are used up.
Among birds that nest in large colonies, such as guillemots, the eggshells may be marked with distinctive patterns, allowing parents to recognize which ones are theirs.
(It turns out that the shell isn’t strictly necessary for the baby bird’s development; check out this video of a chicken developing in a glass container lined with plastic wrap! It’s also a good look at all the parts of the egg mentioned above.)
Eggs may be almost round, or oblong and pointed at one end. It was thought that birds nesting on sea cliffs had pointed eggs so that they wouldn’t roll off into the ocean. This may be true, but further research revealed that the narrow pointed end keeps the embryo properly centered and oriented.
Bird eggs need to be incubated to keep them warm and humidified. Either or both parents will sit on the eggs, depending on the species. They also turn the eggs so each embryo develops properly. Most birds wait until an entire clutch is laid before starting to incubate. This causes the eggs to hatch at approximately the same time.
With nature, there always seems to be an exception. For some species in the family Megapodiddae, the parents build a massive compost pile under their buried nest; heat from the decomposing vegetation keeps their eggs warm, and lets them off the onerous duty of nest-sitting (although the male sticks around to tend the compost pile). When the young hatch, they have to not only break out of the egg (they use their formidable claws to penetrate the shell), but also dig their way to the surface. Other Megapodes use geothermal or solar heat to incubate their eggs.
The next time you encounter a nest full of eggs, stop a moment and ponder how remarkable those little bird-factories really are.
It’s time for the answers to last Monday’s “Name that Bird.” Here is the list again. Would you believe that all but two of these are real? I admit to inventing the Bare-faced Lyrebird and the Giggling Guan. As for the rest of these—no one can claim that ornithologists lack a sense of humor!
- Bare-faced Go-away-bird
- Bearded Guan
- Colorful Puffleg
- Familiar Chat
- Freckled Duck (too much time in the sun?)
- Green Hermit (jealous enough to live alone)
- Horned Screamer (it lives under your bed)
- Invisible Rail (aren’t they all?)
- Large Frogmouth (as opposed to a large-mouthed frog)
- Lark-like Brushrunner
- Laughing Falcon (they like to tell jokes)
- Macaroni Penguin (it’s big on pasta)
- Mysterious Starling
- New Zealand Kaka (kaka meant something else when I was growing up)
- See-see Partridge (this might have come from a birder… “See? See? It’s a partridge!)
- Singing Quail
- Spectacled Tyrant (many tyrants throughout history have worn glasses)
- Spotted Shag (rug?)
- Streaky-breasted Flufftail
- Zigzag Heron