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What Causes a Double-Yolk Chicken Egg?


Annie, one of our Australorps, has always been a big girl who lays big eggs. So when she laid a whopping 4.2 ounce egg one day last week, I wasn't all that surprised.

It turned out to be a double-yolker. Then yesterday she laid another giant 4.1 ounce egg, that dwarfed even our duck eggs. It was also a double-yolker. So I decided it was time to share what I know about double-yolk eggs.


Roughly one in every thousand eggs (about .1%) is double-yolked.  Since commercially-sold eggs in the United States are sold by weight and also candled prior to packaging, any double-yolk eggs are discarded (although in the UK they are not) and you could go an entire lifetime eating store bought eggs and never encounter one.

But start raising your own backyard flock and chances are you'll collect your share. But in nearly five years of raising backyard chickens, and collecting hundreds upon hundreds of eggs, we've only seen a handful of double-yolkers.

However, I recently was contacted by New Age composer Alex Shapiro who told me that she just got a carton of eggs that contained 10 double yolkers! What are the chances of that?

~Annie's huge 4.2 ounce double yolker next to two regular chicken eggs~
So what causes double yolk eggs?

A double-yolked egg occurs when two egg yolks are released into a hen's oviduct too close together and end up encased within the same shell.

Generally about an hour after an egg is laid, the next yolk is released, but due to hormonal change/imbalance, an overstimulated ovary sometimes misfires and releases the yolk too early.  The shell forms around both yolks and results in a single egg. 

~Annie is far larger than our other Australorps~
It's far more common to find double-yolked eggs from new layers or those hens on the tail end of their laying life.  It can be genetic, and therefore hereditary, and is more common in the hybrids and heavier breeds.  In Hong Kong and India, chickens are actually bred to lay double-yolked eggs that are highly sought after by customers.

Double-yolked eggs generally won't hatch if incubated, and if they do, both chicks usually don't survive, although it's possible.  Hens who lay large or double-yolked eggs are more prone to becoming egg bound or suffering vent prolapse, both potentially fatal afflictions.

~a 'regular' Annie egg next to a medium-sized chicken egg~
I admit that I am a bit worried about Annie.

Her regular eggs are huge, weighing about 2.6 ounces, but now that she's started laying gigantic double-yolk eggs on occasion, there's a far better chance she will get egg bound or an egg will break inside her, causing peritonitis.

But there's not much I can do but keep a very close eye on her for any signs of distress and be sure she doesn't get overweight, which can also lead to egg binding.

~Egg weights for standard US size grading~
As you can see from the standard egg size chart above, even Annie's normal 2.6 ounce eggs are considered to be in the jumbo category - and her 4 ounce eggs are off the charts!

She's our best layer and I save her eggs to make fried egg sandwiches for breakfast usually on Sundays. Our smaller eggs I use for scrambling.

~Cracking the first gigantic egg to see if it was a double yolk egg~
~And it was~
If you've never seen a double-yolk egg, you might be interested in watching this short video clip I made:


Annie is three years old, so well into her productive laying life, but hopefully with a few more years of good laying ahead of her. She has the tendency to go broody, and was a great mother to a clutch of chicks this past spring, so either way - as all our hens do - she has a home here with us for a long as she likes.

Update: It's now 2016 and Annie is still going strong.

~Annie raising a clutch of chicks she hatched this past spring~

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