I’ve always wondered how an egg is made inside a female bird’s body. I imagined some organ or gland that handled the whole process and then popped the egg out when it was done. My thinking got complicated when I tried to consider how the bird managed to lay as many as a dozen eggs in a clutch at the rate of one a day. How could this imagined gland complete one egg and then create another one a day later? It must take longer than a day, but wouldn’t egg No. 2 be in the way of egg No. 3, and so on?
I launched into some research and, thanks to my birding buddy and fellow writer Val Cunningham, was directed to “The Avian Egg” by Alexis and Anastasia Romanoff. It turns out that eggs are created in an organ, and that organ is a long tube where a sort of assembly line process takes place. It’s called the oviduct, or egg canal. Totally tubular!
The oviduct has two muscular layers in its walls. The outer layer consists of fibers that run longitudinally along the length of the oviduct. They lengthen or shorten the oviduct. The inner layer fibers run in a circular fashion around the tube and can change the diameter of the oviduct. These muscles help move the ovum and the egg along.
The process starts just outside the entrance of the oviduct at the ovary. The ovary has a cluster of ova in successive states of readiness for launch into the oviduct. Each little ovum begins to accumulate yolk at progressively different rates and, as it reaches the full amount, it breaks away from the ovary.
The ovum enters the oviduct at the infundibulum, or funnel, which is shaped like a lily with “petals” that embrace the ovary and guide the ovum into the funnel, the first of five sections of the oviduct. This is the place where the male’s sperm would meet the ovum and fertilization occurs. Some female birds have sperm storage tubules in this section that allow several eggs to be fertilized in sequence.
Besides the two types of muscles, there are various glands and secretory organs in the wall of the oviduct. And there are folds and creases in the walls that help propel the ovum forward and rotate it as it goes.
After passing through the funnel, the ovum enters the magnum or albumen-secreting region of the oviduct. This is the second and largest section of the oviduct. It has much thicker walls than the rest of the oviduct, walls that hold the glands that deposit the white of the egg around the yolk.
Once the albumen has been deposited, the ovum moves on to the third section of the oviduct, the isthmus, where the shell membranes are secreted. This is a short section, more narrow than the preceding section, hence its name. Two membranes are laid down that will eventually have an air gap between them.
The fourth section of the oviduct is the uterus, where the shell is secreted around the egg. The oviduct doesn’t store any calcium. It’s removed from the blood. If the glands in the oviduct can’t get enough calcium from the circulating blood, they may take it from the bird’s bones, which can weaken the bird’s skeleton.
The fifth and last section of the oviduct is the vagina. Here, the fully formed egg is ready to be laid. The bird can retain the egg until she’s ready to lay it, if, for example, she’s still finishing nest building.
Laying the egg is a bit more complicated than it may seem. The uterus of the previous stage everts, that is, turns itself inside out, essentially unwrapping the egg. The egg does not come into contact with the walls of either the vagina or the cloaca.
That’s important because the cloaca is the multipurpose opening at the end of the alimentary canal (think bird poop) as well as the end of the oviduct. It’s also the place where the male’s sperm gets introduced in what’s called a “cloacal kiss.”
Many birds’ eggs feature spots, streaks, swirls and blotches in a variety of colors. Color starts with capsules of worn-out red corpuscles that rupture. The hemoglobin is released and dissolves in the bloodstream. Then, as the blood passes through the liver, the hemoglobin is changed into bile pigments of red, yellow, blue, brown and black. Finally, the glands in the uterus secrete these colors onto the shell.
Learning about how an avian egg is made has made me appreciate how ingenious the process is and also how arduous it can be for females. Next time you see a female robin with a big tummy, realize she’s probably “with egg” and regard her with some awe and sympathy.
Clay Christensen lives and writes in Lauderdale. His book “The Birdman of Lauderdale” is available at metro Half Price Books stores.