Laura Erickson's For the Birds

Thursday, April 26, 2012

Twins and Double Yolks in Bird Eggs



Every year during nesting season, people ask if it’s possible for wild birds to produce twins--two chicks from a single egg. Chicken eggs from the grocery store occasionally include some with double yolks. One researcher in 1849 claimed to have a chicken that laid 10 double-yolked eggs, which he allowed her to brood--9 hatched, producing 9 pairs of twins. However, this has never been replicated. In one study of more than 1100 chicken eggs, double yolks were found only 3 times--that is, in less than 1/3 of one percent. In another, larger study, 2.8 percent of chicken eggs were double yolked. Very few double-yolked eggs hatch. In that large study, 208 double-yolked eggs were incubated and candled periodically to determine if they contained viable embryos, and eggs were examined after the embryos had died. Over 64 percent of the embryos died within the first 7 days. Two of the eggs--less than one percent, pipped, and only one hatched into twin chicks. There are apparently two peaks of mortality, one about a third of the way through development, and another near hatching--apparently the two chicks against each other have a very difficult time bracing themselves properly to pip the egg.


The yolk contains all the nutrition a chick embryo needs to develop to hatching, so that’s not an issue for birds inside a double-yolked egg. The problem is that two embryos need twice as much oxygen as one, and their growing bodies cause the egg to lose water faster. The pores of an eggshell are designed to provide oxygen intake for one developing chick. And as they grow, each bird blocks the other from properly filling the available space.

Peregrine Falcon

There are two ways a double-yolked egg can form. The hen may ovulate two eggs at once, or she may ovulate properly but one yolk gets stuck in the oviduct longer than usual. When she ovulates next, the second yolk moves down with the first yolk, and both end up together where the shell is formed. A case where this seems to have happened in the wild was in 1981, when an unhatched Peregrine Falcon egg from Greenland was examined as part of a study on pesticides. The egg was both longer and wider than a normal Peregrine egg; its volume 35 percent larger. Inside this double-yolked egg were two dead embryos, one that had died at about 24 days of development, the other at 22 days, so both had reached the fourth quarter of incubation. The younger embryo was more deteriorated and decomposed. The researchers believed that after this embryo’s death, decomposition polluted the egg enough to kill the other embryo.

American Goldfinch

Besides that poor doomed Peregrine egg, there are a handful of known cases of twin embryos in the wild. American Goldfinches, Song Sparrows, and Brown Thrashers have produced twin embryos. There are also several known cases of embryo twins in waterfowl, but only because such a large number of duck and goose eggs have been studied for various reasons--fewer than a half of a percent of examined wild waterfowl eggs have had twin embryos. Interestingly, in several cases the twins weren’t the fraternal twins associated with double yolks, but identical twins, caused when a single fertilized ovum, or yolk, divides into two separate individuals. One 1974 study which included both wild waterfowl eggs and duck eggs inside temperature-controlled chambers found that cold temperatures before incubation could lead to this kind of twinning. None of the embryos in this study survived to hatching.

So twins are possible but unlikely in the world of wild birds. Those grocery store eggs with two yolks are genuine rarities.



Three papers about bird twinning:

17 comments :

  1. This happened years ago but, when I cut open a dozen hard-boiled eggs to make deviled eggs, several of them were double-yolked, giving me the impression that one or more related hens produced them with some regularity. I took 35mm photos at the time, but goodness knows where they are at this point.

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  2. Besides that poor doomed Peregrine egg, there are a handful of known cases of twin embryos in the wild.
    Thanks for sharing this article; it’s been a really great interesting read.

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    1. Double yolks aren't that rare actually. My family always buys Jumbo eggs at the store and there's usually one double - yolker per dozen. I've rarely ever seen a double yolk in the smaller sizes though, maybe twice ever.

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  3. Yep. Double-yolked eggs aren't that rare. But twin hatchling birds are.

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  4. I have a Cockatoo egg in an incubator right now and there are twins alive inside of it. I am freaking out because of the low percentage of survival. They are 12 days old and doing very well. Both birds move interdependently and are frisky. It breaks my heart of their sure demise. I wonder if there could be a way to inject air into them. Does anyone have any ideas?

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  5. I have no idea how to help. Maybe a vet school that specializes in captive rearing parrots? Best wishes.

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  6. Just had 2 healthy twin pigeons from 1 egg..what are the odds of that?

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  7. Hello, we have had this just happen with Robin's in our back yard. I had been taking pictures all along side the eggs are laid and there was three. After a couple days after having I took another picture and it looked odd but just thought they were laying a bit goofy. Then last week since they are just above me being able to see in i counted by touch to make sure all were good. There is a darn cat roaming around and wanted to make sure they were ok. ( And before anyone trashes me who may not know, touching them resulting in them being abandoned is a wives tale (25 yrs SPCA Inspector and past president of a wildlife rehab). So, when I touch counted suddenly WAIT there is four. I went back and looked at the egg pics and yup only three but one was a bit later than the others. Well they are soon to be getting ready to start off in flight school and I am still proud father of four. Pics to prove, yup it happens in the wild and survived.

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  8. I was wondering if these twins were identical or fraternal, i.e. resulting from one embryo separating into two VERY early on, versus two separate embryos. The given description strongly implies egg twins can only be fraternal.

    Is this true?

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  9. This morning I had twin ducklings hatch from one egg. They are both alive and doing well. I wouldn't have believed it if I didn't see it myself. I was helping the first duckling out of the egg as it seemed to be struggling, and once it came out I realized there was something heavy in there and when I looked in the rest of the egg there was another duckling. First, I didn't think it was alive but when I pulled the egg out from around it there it was! I am still in awe and looking for information on how often this happens.

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    1. Sadly (my twins) were feral pigeon chicks nesting on scaffolding close to my kitchen window and did not survive. One was taken by a cat or a rat at about a week old, the other fell off the tiny ledge. I hope you got photos of the hatching - or people wont believe you - it is THAT rare. I didn't see my hatching properly - but there had been 2 eggs - 2 chicks - and an unhatched egg. I posted the pic looking for some info in a pigeon forum and was accused of putting an extra egg in the nest for publicity/attention- they were wild pigeons.

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  10. When I had it happen with pigeons I looked it up and sure it was something like one in 40,000 double yolk eggs result in 2 healthy chicks. Usually one or both die in the egg (lack of space) or there is severe deformity

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  11. I sought out this artical because I bought a carton of hens eggs extra large, I cracked open three eggs. All three were double yolks. I have nine to go.

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  12. I have 2 parakeets that laid 6 eggs. This morning I awoke to a big surprise. Out of one egg, emerged two chicks! Two healthy, peeping, Budgie babies.

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  13. We have had a pair of Say's Phoebes that have nested on a ledge on our porch for the last 5 years. I don't know if they are the same pair or the offspring of the original pair. For 4 years they had laid and hatched 4 eggs. This year they came early and laid and hatched 4 eggs again, but as soon as the babies left the nest the mama bird laid another 4 eggs. My husband climbed up on a ladder to see if they really were sitting on eggs again and he counted 4 eggs. They hatched in late June and the parents were looking unusually frazzled and seemed to be working overtime to catch bugs for the little peeps. Five days ago they outgrew the nest and they lined up as usual to look over the edge of the ledge and lo and behold there were 5 baby birds all in a row. I had assumed that the parents were looking worse for wear because they hatched 2 nests full of babies, but when I saw there were 5, and one was slightly smaller than the others, I wondered if wild birds ever had twins from one egg. That's how I ended up coming here to find out if it was possible. I had noticed that the parents were feeding the littlest one more often than the others, at least when they were lined up like that. They are interesting little birds and since I sit out on my porch for hours a day, they had been landing on the back of a chair across from where I sit to show me the bugs they caught before flying up to feed the babies. This year was extraordinary for them, both for the second round of eggs and because it appears that they had twins! I didn't think it was possible, even though I have seen a fair number of double yolk chicken eggs. I never heard of chicken twins and I just assumed that the embryos would die before they hatched. Learning something new every day!

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