Magic in Nature: Fairy Eggs

About a week ago, I was happily collecting eggs when I came across this one.

Here it is with other, normal sized eggs and their corresponding weights in grams.

I had heard of these eggs, but had yet to see one or have one of my hens lay one. These little eggs are known as fairy eggs. They have a multitude of other names, such as rooster eggs, wind eggs, witch eggs, and fart eggs. Normally, young pullets (females that are just starting to lay eggs) will lay fairy eggs, however, it is possible for older hens, too. In my case, this egg came from a three year old hen. In terms of her egg laying years, she is an old hen and nearing the end of her productive life. So what makes a fairy egg a fairy egg? It’s really what they’re lacking. These eggs are missing a yolk. Because the yolk is missing, something else had to trigger the egg production process. So let’s pause here on fairy eggs and take a look at what egg production in the hen is like.

In the normal egg laying process, the yolk being released from the ovary causes the rest of the reproductive tract to add the other parts of the egg and then the hen lays the egg. Repeat.

Now we are speaking my language! This conversation is a reproductive physiologist’s dream! Or maybe just this reproductive physiologist. 😉

The reproductive tract of the hen is called an oviduct. This is different in mammals where the tube connecting the ovary to the uterus is called the oviduct, but there are other parts beyond the oviduct. That’s because the hen doesn’t need a uterus for gestation because the chick develops in the egg.  So we can think of her tract as a tube that connects her ovary to the outside of her body. Now if we look at the word oviduct, translated from Latin, we have “ovi-” and “-duct”. “Ovi-” is translated as egg, and “-duct” is translated as tube. The oviduct is literally the egg tube. Make sense? Great!

Another difference between the reproductive tracts of birds and mammals is that while they both started out with two ovaries and two oviducts (and uterine horns in mammals), only the left side develops and is retained in birds, so they have one ovary and one oviduct.  See the photo below to see what the reproductive tract looks like!

The egg production process starts at the ovary. The ova, or eggs, develop as yolks on the ovary. Each yolk is released through a process known as ovulation. This happens every 26 to 28 hours.

Have you heard the word ovulation before? Probably. It’s something that happens in every female mammal, the ovary releases ova in hope of meeting sperm at the site of fertilization and producing an offspring. In the case of women, ovulation happens every 28 to 35 days. Horses, cows, pigs, every 21 days. Anywho…..

As the ova (yolks) develop they get bigger. If you see in the picture they are labeled as “F3”, “F2”, “F1”? As the time for ovulation nears, the largest yolk will be ovulated. In this picture, F1 would be the next to be ovulated. Once ovulation occurs, the yolk is released from the ovary and is caught by a funnel shaped structure, known as the infundibulum. The infundibulum acts like a slow-moving catcher’s mitt more than a funnel and moves to surround the yolk and ensure that it gets inside the reproductive tract. If fertilization were to happen, it would happen in the infundibulum. But that’s another story for another day.

After spending about 15 minutes in the infundibulum, the yolk moves into the magnum. This is the largest portion of the oviduct and the yolk spends about 3 hours here while the thick albumen is added around it. It is during this time that the chalazae are developed and added to the egg as well.

Next stop is the isthmus! Now for those of you geography nerds out there, you know that an isthmus is a thin strip of land that connects two larger pieces of land. That’s how this isthmus got its name. It is a thin section of the “tube” that connects larger sections on either side of it. Here, two shell membranes are placed around the thick albumen and egg yolk. These membranes are your nemesis when peeling hard boiled eggs. This only takes about an hour and it’s off to the next stop!

The shell gland is where the thin albumen and the egg shell are added. This takes the most time of about 20 hours! After that hard external shell is applied to the egg, the egg then moves into the vagina and will be laid, exiting through the cloaca, then vent. If you pay attention to the picture above you will see that the large intestine is also attached to the cloaca. That’s because the urinary, digestive, and reproductive systems are leave through the same structure in the in bird! All three of those organ systems empty into the cloaca and pass out of the bird through the vent, which is the external closure of these systems. Once the egg is laid by passing through the vent, the hen will ovulate again in about 30 to 75 minutes. How cool is that!

Now this all begs the question, do birds lay eggs every single day?

Well, what are your thoughts?

Generally speaking, yes. But I’m not good at generalizations. No, they do not lay every single day. They lay most days. But hens are naturally sensitive to light and their reproductive tract responds to that. If a hen lays an egg too late in the day, the next ovulation will be put off until the next day when more light is present instead of within the 30 to 75 minutes later I mentioned. So in this case, the hen would not lay an egg the following day. And that’s ok, because even when this happens the hen still lays an average of 270 eggs per year. That’s about 23 days a month, which is more days a month than most of us work!

Now that you have allowed me to teach you about reproduction, which is my heart’s desire, back to fairy eggs.

Of course, I had to crack open my fairy egg and see what was inside. In the following picture, you can see a few things. You can see two types of “fluid”, one thicker, more gelatinous, the other thinner, more watery. The thicker portion is outlined in teal in the photo below. These two “fluids”, if you will, are the albumin, or the white of the egg when it is cooked. The part closest to the yolk (where the chick would be developing) is thicker, while the fluid in the rest of the egg is thinner. You can see each “fluid” labeled here. This difference in fluid viscosity is similar to that of mammals developing the placenta as well, but I digress…

You can also see two white stringy things (see arrow). These are the chalazae they are attached and added to the egg with the thick albumin. These act as suspension for the yolk and help keep it suspended within the egg. Crack open any egg and you’ll see these.

The last thing I want you to take note of is the object circled in red. This is likely what triggered the formation of the egg without the presence of a yolk. To the naked eye, which is how I viewed this, it appears to be a sloughed off piece of tissue from the hen’s reproductive tract. This is normal as the lining of these tissues routinely replace themselves with newer cells, as does every organ system in their (and our) bodies. Once this piece was sloughed off, while it appears small here, it was large enough to trigger egg production just as a yolk would.

This has been my one and only fairy egg in my over four years of chicken farming. My hen is back to laying her normal large size eggs and is perfectly fine and healthy, just aging and undergoing cellular renewal.