Meat bird breeds

Knowledge, or in the case of hens that successfully hatch out double yolkers, maybe instinct. Hardly guaranteed death.
 
I get the whole concept of how long the process takes. But if chickens only ovulate once per 24 hours, how come we get double yolk eggs? Or is that another myth that never happens according to the experts? Is it not concievable that if a chicken can drop two yolks in one shell, that they couldn't drop two yolks in two shells. My chickens have large bones. Their bones are typically marrow filled and not hollow. Their eggs tend to be very hard shelled, similar to guineas, if you drop them off the counter, they may not break when they hit linoleum. I have never seen a double yolk, but I have seen two eggs in a pen with one hen, MANY times.
I have to say that many things are possible.

Perhaps eggs were lost in the bedding. There are all sorts of explanations more likely than that a hen would actually have the ability to lay 2 in a 24 hour period.

Ova mature in sort of an assembly line process but the speed at which they mature varies.

Ovulation occurs when an ovum matures and drops into the infundibulum. Sometimes there can be 2 mature enough to be released and most often happens in pullets before their reproductive systems have their timing worked out.
But consider that roughly 0.1% of eggs have 2 yolks so it is extremely uncommon. Hormonal imbalances or genetic defects cause that overstimulated ovary. It is not normal. Double yolks sometimes cause egg binding or even prolapse so can also be a death sentence for the hen.

As far as dropping 2 yolks in 2 shells. That's not how it happens. The yolk or, in the case of double, triple, etc., the yolks pass through the infindulum, magnum, and isthmus before entering the uterus. In those first 3 steps the inner and outer membranes, water and albumen are added in layers of secretions that take about 4 hours.
At that point, the egg enters the uterus where the shell is built around those components. It takes at least 20 hours to apply the calcium carbonate around the membranes. The uterus is about 4 inches long and IMHO, there isn't room for there to be 2 shells to be built in there at the same time.
So why is a double yolk a "death sentence"?
Mammals and other live bearers have abdomens that swell to accommodate whatever is in there. They also provide the nutrition demanded by the embryo/s up to a point.
An egg contains all the space and nutrition there will ever be. Space and oxygen are the big issues. Each twin in a bird egg has its own yolk but they also get nutrition from the albumen, which they have to share.
It is a death sentence in well over 90% of the instances when they are incubated.
In the extremely rare case that both would survive to hatch, they won't be as vigorous as non-twins. Why push that, just fry them up and enjoy.
If you choose to disagree, go ahead and incubate all your double yolkers and let us know your success rate.

Knowledge, or in the case of hens that successfully hatch out double yolkers, maybe instinct. Hardly guaranteed death.

Likely guaranteed. Have you ever seen a hen hatch two chicks out of a single egg?
 
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Yes I have seen a hen successfully hatch two chicks out of one egg. Hens are a little more skilled than we humans. They don't always make it and more often one dies, and the odds are definitely against them but under a hen retaining some degree of natural instinct it can be done. I know this because I shoved a bunch of double yolkers up under a hen. Telling people that double yolks are a death sentence to the chicks inside is not entirely accurate. If someone reading was working with a rare critical breed, they might toss eggs they though were double yolk, (which might in fact just be big eggs), when in fact there is a chance they could have at least a small chance of success.

As for your explanations, they hold no merit. I have birds in 5x5 pens. One hen, one cockbird. They are basically on lawn grass. When they start to lay, there is usually no nest box present. When present, a nest box is a five gallon bucket, to begin with with no bedding at all. I move pens every day. No places for eggs to hide. In birds that I am interested in keeping offspring out of, I write on eggs which pen they came from and a date. So the only possible explanation is that the information that you have does not take into account all of the physical differences in chicken breeds. I know from my research that my birds have much longer intestines than other breeds, so I guess is conceivable that they have a longer egg tract as well. I think that they may be able to hold their eggs, because often, when I see two a day, there is a cold snap that interrupts laying, when warm weather resumes, you get two eggs. I have seen two eggs during periods of uninterrupted laying, however.
 
Meant to say they have shorter intestines, which would leave more room. Also they are white, and the darker the egg, the longer it takes.
 

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