How a chicken lays an egg

February 17, 2009 | By | Comments (7)

“The acme of food packaging.”

—Alan Davidson, The Oxford Companion to Food (Oxford University Press, 1999)


Ophelia in the nest box, with her blue egg (the other is a decoy). Photo by Elizabeth Jardina

Every day, our chickens deliver eggs to us—anywhere between two and six, depending on the weather (in winter, they slow down). Even though we’ve been collecting them for over a year now, we can’t quite take them for granted. Each egg is slightly different in shape, or color, or thickness of shell, and still seems faintly miraculous to me. I mean, put yourself in the hen’s position. Reproducing yourself every day (or trying to, at least) is no small feat.

It’s pretty hard to catch a chicken in the act of laying an egg, if you have a day job. The thing pops out in less than a minute (although we did get close with Ophelia, above). If you’re really curious, watch this video, courtesy of a Barred Rock owner.

What about the preamble, though—the formation of a bulky egg with a shell on it, within the chicken?

Here’s how it works.

Sketch courtesy of University of Illinois Extension

Every female chick is born with thousands of undeveloped yolks, or ova, grouped together near the middle of her backbone in a larger cluster, the ovary.

When a hen is ready to lay, these ova begin to mature, and every 24 to 26 hours, a fully formed egg yolk is released into the oviduct. As the yolk moves down this tube, it’s coated with layers of gel-like albumen (that’d be your egg white) and wrapped in a thin, translucent membrane. If a rooster had been on the scene, sperm would probably have fertilized the yolk before it met the albumen.

Then comes the amazing part. As the soft, shell-less egg moves toward the exit, it passes through a floating cloud of  calcite (calcium carbonate).

The egg’s membrane, which I’d thought of mainly as that annoying film that you have to peel off a hard-boiled egg, is actually pretty wondrous. All over its surface are precisely spaced protein points. These attract the calcite particles, which build up on the membrane in crisp, geometric columns until they make a shell. Essentially, they form a thin crystal that covers the egg. How appropriate that Fabergé used eggs as the model for his fantastic creations.

Shortly afterward, the hen gets an urge and climbs into the nest box. After a bit of heaving and panting, out pops the egg. There it is, protein-rich, marvelous, ready to go. We take it away to eat, and the hen starts making another one, as if it were the most ordinary thing in the world.


  1. Linda

    As I kit my dad had vhickens in a battery cage where they would lay the eggs. I always watched as the eggs come out and sometimes would touch the egg as it comes out. The shell was still shoft but hardened as it came in contact with oxigen. Is this true? Or does the shell harden totally inside the chicken?

    January 31, 2017 at 5:21 am
  2. Heather

    Thanks for this great info! I raised chickens through high school (not normal for most kids at that age) for both eggs and meat.

    This summer I got back into having two hens and a rooster for my husband and I. Since it’s just the two of us, two eggs a day is perfect for the weekend breakfast and just to have a few eggs around.

    But the other day I found my first two “soft eggs”. They came later in the evening after two normally formed eggs.

    It was really strange but this answered my questions as to why they came out like that!!

    July 7, 2009 at 12:49 pm
  3. Suvir Saran

    Thanks for this post and the photo and the video Margo.
    Like with your food writing, you do such a great job sharing the nitty gritty of egg laying for chickens.

    We have a flock of 80 plus chickens and geese and ducks. This last week has brought us our first geese eggs for the season, and also our first duck eggs ever.

    Wish you great joy as you care for the chickens. And keep your posts coming.


    March 13, 2009 at 5:08 pm
  4. Margo True

    Larry, we’ve had quite a few of these weird goopy eggs, especially when our hens were just starting to lay. Feeding them crushed oyster shells (for extra calcium) really helped; we try to scatter at least a cupful in the henyard every other day. You can buy oyster shells at any pet store

    March 5, 2009 at 3:44 am
  5. larry reynolds

    Apparently an egg I got today missed the cloud altogether. The egg is normal in appearance but is missing the shell. Yep, it feels like a balloon in the shape of an egg. Whaat’s with this??

    March 2, 2009 at 12:20 am
  6. Margo T

    Kathy G, we’re so glad we can spark your wish to explore chickens and/or bees. As you’ve probably gathered, the chickens are a lot less work than the bees…they’re practically upkeep-free, in fact, and give you the daily gift of eggs.

    Bees are more fragile, but those of us on Team Bee are utterly fascinated by our hives; the honey they give is just one sweet dividend of what has almost become a daily meditation on the State of the Bee. Either project is hugely rewarding, and I hope you do start one or the other, or both. You won’t regret it.

    February 18, 2009 at 4:24 am
  7. KathyG

    Wow, this is good info! I love the ‘floating cloud of calcite’ — sounds rather ominous, like some alien menace from a bad sci fi movie. I really enjoy reading about your chickens (also the bees, the vegs, and the whole one-block diet concept). I hope-wish-fantasize about having a hive of bees and a few chickens of my own someday, here in my own garden. You inspire me to take more seriously my own ability to grow my own food and think of my little plot here as a real mini-farm. My climate is much more challenging than yours (Sunset Zone 1) but after gardening here for 35 years I have learned a few tricks and do much better than the average local gardener. I think I’ve grown complacent, though, with my tomatoes and paprika peppers, and your blog is a new source of inspiration and challenge to me. I’ve been a subscriber to Sunset (the magazine) for almost 40 years, and in many ways this blog is the best resource I have. Thanks so much for writing it!

    February 17, 2009 at 2:14 pm

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