It’s All in a Day’s Work!
November 3, 2025
EggTober 2025 has officially ended! And Wow … I’m so eggs-cited to share Submission Four, the finale, with all-y’all! The eggs showcased for Days 25-31 +1 (yes, another random bird flew in and laid a bonus clutch on the last page) are all snug in their New Mexico nests. Also, you’ll notice a bit more text surrounding those nests.
Following Submission Three, where my narrative focused on the different layers of the eggshell membrane, I was curious about how avian eggshells are constructed. Then I wanted to know more about the entire reproductive system of birds, from ovulation to fertilization, to egg laying. And Then I couldn’t rest without knowing about the embryo, the yolk, the egg white, and all the bits and pieces you see when cracking open an egg! I didn’t know any of this, and if you don’t know, you’re head is about to explode in wonder!
The Avian Oviduct and Egg Formation
It takes about one day to build an egg. But to prepare for the journey, about 7 to 9 days before the egg will be laid, the Yolk must be formed. Here’s the story of …..
The Yolk …..
….. formed in one of the many Follicles of the Ovary, the vitellus or Yolk begins as an immature Ovum that is stimulated to enlarge over several days by receiving deposits of yolk material. Once growth is initiated, over the next 7 to 9 days the Yolk’s formation intensifies as 99% of its nutrient-rich layers are deposited until it’s fully formed. Now mature, the Follicle ruptures and the Yolk is released in a process called Ovulation.
And this is where the Journey through the Oviduct begins! Over the next 24 hours, the developing embryo acquires the many layers it needs for nourishment, respiration, and protection until it’s formed into a perfect egg ready for laying. The entire journey takes place in the Oviduct …..
….. which can be divided into several regions: the Infundibulum, Magnum, Isthmus, Uterus, and Vagina. What happens in each of these regions is nothing short of fascinating!
Infundibulum – After Ovulation, the Yolk immediately enters the funnel of the Infundibulum (the Ostium) where the Ovum is fertilized and the Chalazae is formed. With the help of ciliary action, it takes 30 minutes for the Ovum to move through this region of the Oviduct to the next, the Magnum.
Magnum – It’s this region where the Yolk, together with the now developing embryo and the Chalazae, gets a protective coating of protein-rich Albumen, a process that takes about 3 hours to complete, before moving along to the Isthmus.
Isthmus – The Inner and Outer Shell Membrane Layers and the Nucleation Sites are formed in this region of the Oviduct. This takes about an hour before the package is delivered to the Uterus.
Uterus – This is where five of the six layers of the outer Eggshell are formed. They are the Mammillary Layer and Mammillary Bodies, Organic Matrix Layer, Crystalline Palisade Layer, Vertical Crystal Layer, and Shell Pigment Layer. Also, as much as one-third of the protein in an egg is added while in the Uterus.
It takes about 20 hours for the egg to move through the Uterus while these layers are deposited. And as the egg moves, it twists and turns. When it’s time to add pigment, the rate of rotation adjusts to ‘paint’ the species-specific patterns we see on the eggshell, leaving a visual record of the egg’s movement in the Uterus. For example, if streaks or elongated tracks on the shell are required, movement is more rapid than when creating round spots, blotches, speckles, or bands. After pigment is added to the outer shell structure, the layers of background color and any markings are enclosed in calcite crystals.
Vagina – The last region of the Oviduct where the Cuticle (the Bloom) is added to the shell just before the egg is laid.
The egg is now complete. The entire process, beginning with the release of the mature Yolk and Ovum from its Follicle until the egg’s final touches, takes 24 hours! And the timing ensures the egg is always laid sometime during the day.
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Glossary and Other Interesting Things
Albumen – refers to the “white” of the egg. The Albumen is an effective barrier against microbes that might cross over and cause disease in the developing embryo. Albumen contains over a hundred antimicrobial proteins.
Chalazae – two spiral bands of white, stringy tissue that suspend and hold the Yolk in the center of the Albumen.
Crystalline Palisade Layer – A tough, mineralized structure of calcium carbonate crystals that grow into dense columnar units above the Mammillary Bodies (the mineralized base of the crystalline eggshell). The amount of shell deposited is determined by the time spent in the Uterus.
Eggshell – Depending on species, the eggshell represents from 9-15% of the total weight of the egg.
Eggshell Formation – The Crystalline Palisade Layer is essential to the process of eggshell formation which takes place in the Uterus.
- Nucleation: The process begins at the Nucleation Sites, located on the External Shell Membrane, which are the specific places where the mineralization of calcium carbonate begins.
- Initial growth: As calcium salts are deposited and crystallize, inorganic calcium carbonate crystals grow around and outward from the Nucleation Sites, forming the cone-shaped Mammillary Bodies; the first, innermost calcified layer of the eggshell, anchored to the External Shell Membrane. This base of mineralized shell provides mechanical strength, and serves as the main source of calcium for the developing embryo’s skeleton.
- Maturation – The calcium carbonate crystals continue to grow into the dense columnar units that form the Crystalline Palisade Layer, creating a tough, mineralized structure that protects the embryo. The amount of shell deposited is determined by the time spent in the Uterus.
Follicle – an enclosed cavity in the Ovary
Labile Medullary Bone – a temporary, highly porous, and woven bone tissue that forms, driven by hormonal changes in female birds, in her medullary (marrow) cavities in the period leading up to and during egg-laying. As a highly porous, woven bone, medullary bone has no significant mechanical function or structural strength. Its role is strictly metabolic.
The word labile means the bone is unstable and readily undergoes rapid and frequent change. Once the egg-laying period is complete, hormones change and medullary bone is reabsorbed.
Labile Medullary Bone Formation and Calcium Requirements – It’s interesting to note that an egg-laying hen requires 10% of her the total body calcium reserves during each 24-hour period she is producing eggs. To meet this calcium requirement, primarily for eggshell formation, it’s necessary that her plasma (blood) calcium levels triple during egg formation. This calcium is mainly obtained from increased intestinal absorption and a highly labile reservoir found in the medullary bone. In other words, to meet the calcium requirement, the hen’s body naturally produces it as medullary bone.
Mammillary Bodies – The first, innermost calcified layer of the eggshell, anchored to the External Shell Membrane. These cone-shaped Mammillary Bodies form the base of mineralized calcium carbonate which provides mechanical strength to the shell, and serves as the main source of calcium for the developing embryo’s skeleton.
Nucleation Sites (aka Organic Cores) – form in the Isthmus region of the Oviduct, and are found on the External Shell Membrane. Composed primarily of organic material (proteins, proteoglycans, collagens), they are the specific locations where the mineralization of the calcium carbonate shell begins. In other words, the Nucleation Sites provide the initial template or “seed” for calcium carbonate crystals to start forming.
Oviduct – the tube that transports the developing egg with embryo from the Ovary to the Vagina.
Oviduct … Only One? – Yup! Most bird species have only one Ovary and adjoining Oviduct, the other having degenerated when the hen was, herself, a developing embryo. This evolutionary modification probably resulted because egg production from two ovaries would deplete the female’s body of calcium to excess. As has been demonstrated in chickens that are on a calcium deficient diet, egg production ceases.
Ovulation – The process in which the mature Yolk and Ovum is released from its Follicle in the Ovary and is received into the Oviduct through the Infundibulum.
Ovulation Rate – Within 1 hour after a hen has laid an egg, the next mature Follicle in the Ovary ruptures (aka ovulates), releasing the mature Yolk and Ovum.
Ovum – an unfertilized egg
Pigment or Not? – Whether an eggshell is white, or decorated with a background color and/or markings, it’s appearance is influenced first by the species of bird, then by lifestyle and nest location. The specific color of an egg is an adaptation to its environment. This is where camouflage and/or thermal regulation may be required.
- Thermal regulation is still being studied, but where camouflage is beneficial, egg color depends strongly on nest locations:
- White eggs – prevalent among birds like pigeons, doves, swans, many seabirds, etc. These birds often build concealed nests where camouflage isn’t necessary, or inconspicuous locations are chosen for nesting where white coloring helps camouflage the eggs, reducing the risk of drawing attention.
- Blue or green eggs – Bluebirds, robins, sparrows, parrots and other birds that lay pale to bright blue, blue-green or green eggs blend in with the sky or the foliage of nesting sites, providing natural camouflage.
- Brown or speckled eggs – Brown or highly marked eggs with speckles, spots, blotches, or scribbles provide excellent camouflage for ground-nesting birds like quail, avocets and killdeer. The mottled appearance of these eggs blend well against rock, sand, soil, foliage and branches. The eggs’ colorations are camouflaged well in nests woven from a variety of materials to nests that may be nothing more than a scrape or depression in the ground.
Shell Pigment Layer – Pigment granules are deposited on the outer shell structure, forming color layers which are then enclosed in calcite crystals.
Sperm Storage – a female bird need mate only once for the sequential formation of her eggs to be fertilized. In other words, each newly ovulated egg that arrives at the Infundibulum, which occurs every 24 hours (more-or-less), becomes fertilized from a single mating. That’s because female birds can actually store sperm in Sperm Storage Tubules (SSTs). SSTS are tubular “invaginations” in the Infundibulum where sperm can be kept alive for 2 to 15 weeks (depending on the species), and can be released after Ovulation.
Uterus – the Shell Gland
Yolk – The nutrient-bearing portion of the egg containing most of its fat, minerals, and many of its proteins and blood vessels.
Yolk Behavior – The Yolk always rotates so the developing embryo floats to the top, regardless of the egg’s position
Zygote – the fertilized Ovum
Wow, gosh! I’m egg’s-hausted ….. how about you!?! But wasn’t that a fascinating journey through a bird’s oviduct? A literal look behind the scenes!
And that concludes my Inktober/EggTober 2025. Researching so much information not only took me down some fascinating rabbit trails, but everything I learned in the month of October blew my mind! And everything I uncovered had to be shared with you all. The bounty was voluminous, resulting in each Submission eggs-panding to accommodate nearly everything. Yes, you read that right ….. I nearly got everything shared, and there’s still so much more to learn about past, present and future bird eggs and all eggs in general. Maybe once my notes are gather and organized, and I pursue answers to countless questions on the when’s, why’s and what-for’s, it will be time for EggTober 2026!
Did you participate in this year’s Inktober? Maybe my four submissions gave you ideas for Inktober 2026? Share your thoughts and ideas. Meanwhile ….. That’s all for now, yolks!
As always, thanks for stopping by!
P.S. in case you missed any of my previous EggTober 2025 Submissions, you can catch up with the following links: Submission One, Submission Two, and/or Submission Three! Enjoy!
a curious nature 