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What affects egg size? What about double yolks?

I was asked about this in a text today, and rather than write back a long text, I decided to answer here. This could also be in the FAQ, but I will just link to this post.

Eggs come in all sizes and there are many factors that come into play. The genetics of the breed comes first to mind. A goose’s first “pullet” eggs will still be extra, extra large on the chicken scale. They are big birds and put a lot into each egg, so it needs to be large. Some chicken breeds are known for laying extra large eggs. And some for laying small eggs.

Pullet eggs are those smaller than normal eggs that many hens start with. Especially the early laying breeds and hybrids, some skip this and wait a few more weeks before starting to lay, but start with large eggs. This is also genetic and breed correlated, but some individual pullets will start laying earlier or later than the norm for their breed.

Sometimes a hen (or more often a pullet) will lay an extra large egg that contains 2 yolks. Usually referred to as a “double yolker”, these are essentially mistakes in the bird’s reproductive tract. They usually skip a day before laying one, so you aren’t really getting more yolks, just 2 in 1. Breeders never select for a propensity to lay double yolk eggs because they rarely hatch (there are exceptions – seems like every one is documented on YouTube). Because you are not really getting more egg (due to the skipped day), they are not the bonus they might seem to be. It also seems like it would be hard on the hen to lay the larger than normal eggs.

If you want the largest eggs, be sure to select breeds or hybrids known for laying large to jumbo eggs. Commercial hens are bred to lay a consistent size large, as any other size does not make as much money per pound of feed consumed. The larger heritage breeds are your best bet for the largest possible eggs, or hybrids of those large egg breeds.

Hybrid vigor can also play a role. The largest chicken eggs on my farm right now are from some Olive Egger project hens that are retired from breeding, but still laying and some of them are laying enormous eggs. The really large eggs from these hens no longer hatch well, but they sure bulk up a carton of eating eggs.

As far as I know, breeding for exceptionally large eggs is likely to produce poor hatches and possibly limit the lifespan of the hens, neither are choices I am prepared to make as a breeder.

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Blue Egg Genetics in Legbars

It is not a secret that most of the breeders of Opal Legbars are struggling to have all blue eggs. This is especially problematic because the gene for white eggs in recessive, so it hides easily in a flock. It does not help that the roosters never lay any eggs, blue or white. The only way to ensure you have eliminated the white egg gene is to have the bird genetically tested. That gets expensive, fast. My flock coming into 2023 was all sired by a male that was known to have no white egg genes, but the mothers could have 1 or 2 copies and still lay blue eggs. Statistically, at least half will test as having 2 copies.

I started by testing the 5 cockerels, 3 were found to be heterozygous and were removed. The remaining 2 have sired all the Opal chicks since 2/15. The pullets are all laying blue eggs, but could have just 1 copy, so at least half should have 2 copies of the blue egg gene. That means up to half could have 1 white egg gene. For a breeder to buy and test these chicks is costly. Tests are $20 to $25 each and you will likely find half the results are not the genetics you are looking for, making each “good bird” you find cost about $50 in testing alone. That is the financial basis for charging $50 per pair for the chicks from the “tested as true blue” flock.

But there is a cheaper alternative for breeders to be able to offer Opal chicks in spring 2024 (and beyond) that they know will lay blue eggs. This describes how to do this as economically as possible.

  • Purchase the desired number of Opal pullets for $10 each. These will have 1 or 2 copies of blue egg gene, but it does not matter, as you will get that from the cockerels.
  • For every 8 to 12 pullets, purchase a “true blue” Opal cockerel for $25. These are the key to making sure all the chicks you sell will lay blue eggs. One cockerel can father dozens, even hundreds, of chicks over the season, making this very economical.
  • If you want to also offer non-opal legbars, purchase some Cream legbar pullets as well. They will live with your flock of Opals and be mated to the same Opal cockerels. You will not be able to tell their eggs apart, but the chicks are easily differentiated, so you can sell both colors from a single pen.
  • To plan your replacement flock (producing chicks in 2025), keep some of the non-opal chicks from the Creams you added, male and female. These are true blue as their cream mothers also had 2 copies of the blue egg gene. When you breed the F2 generation from these, all chicks are true blue and always will be in future generations.
  • Only about 25% of the F2 generation will be Opals, so save some of them and build your future flock of Opals from the F2’s. You can also save some of the non-Opal siblings so in 2026 and onward you can still produce both colors from a single pen, all homozygous for the blue egg gene.

This plan will allow you to build your own flock of true breeding Opals, while producing salable chicks each year to pay for the feed. The alternative is to wait and buy true blue chicks in 2024. They will be much cheaper then because my entire flock of Opals will be producing them. But you will miss the entire spring 2024 sales season, which will more than pay for the costs of buying some pricier cockerels in 2023.

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Statistical probabilities with the blue egg gene

For a lot of my project birds, I am working with the blue egg gene to create blue or green eggs. A green egg is just blue, plus some brown color. This gene is tricky to work with for 2 reasons:

  • It is dominant – that means that a hen that lays a blue egg might only have 1 copy of the blue egg gene and so could make a chick that carries no copies.
  • The cockerels never lay eggs, so you can’t tell if they have even 1 copy of this dominant gene

Short of paying for a genetic test, the best I can do is to compute probabilities of having at least 1 copy of the gene. Because there seems to be a shortage of layer chicks this year, and the pullets that I am finished with from my Isabel Welbar project are such great layers, I decided to put them into my Opal Legbar pen. Until last week, there were 5 cockerels in there. Two are homozygous for the blue egg gene and the other 3 are hetereozygous. Some of the pullets lay olive eggs and some lay medium brown eggs, about half and half.

So, for all of us that loved word problems in high school math, here is a chance to apply that. Statistical probabilities are portions of 100%, and this is a binary possibility (lay green eggs or not). So, I’m going to compute the chance of getting non-green and subtract that from 100%.

60% of the fathers have 1 copy of the white egg gene (non-blue), they will have a 50% chance of passing that to their progeny, so the chance of getting a white egg gene from the father is 30% (.5 x .6). The chance of getting a white egg gene is 100% from the brown egg pullets and 50% from the green egg pullets. Given half of the pullets lay brown, the chance of getting a white egg gene for the mother is 75%. To lay a non-green egg, the chick must get a white egg gene from both parents (white is recessive to blue). So we multiply the 2 probabilities (.75 x .3) and get 22.5% (chance of non-green). Subtract that from 100% and we get a 77.5% chance of each chick laying a green egg.

On February 15, I removed the 3 Opal cockerels that have a copy of the white egg gene, leaving only the 2 that have 2 copies of the blue egg gene. It takes some time to ensure the hens are not retaining sperm from the removed cockerels, but once that passes, the probabilities change. Since only 1 copy of the blue egg gene is needed to turn the daughters eggs green, the chance of green eggs will go to 100% very soon.

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Genetic testing of chickens

I have started using a genetic testing laboratory in Florida to test some of my breeders for the blue egg gene. The process is easy, but not cheap, and results can be disappointing. This last batch I tested all 5 of my Opal Legbar cockerels and 2 of the best looking pullets. The entire flock was sired by a genetically tested cockerel that was homozygous for blue eggs, so the expected results were to get at least half the flock to test homozygous (meaning they have 2 copies of the blue egg gene and so will breed “true” for blue eggs).

Of the 3 cockerels, only 2 were homozygous. I have since removed the other 3 to the “bull pen” where extra roosters live. This means that soon, all the eggs I collect from the Opals will produce chicks that have at least 1 copy of the blue egg gene (and therefore lay blue eggs, as that trait is dominant over white eggs). The conclusion from all this is that starting in April, every Opal pullet chick that hatches will have a (nearly) 100% chance of laying blue eggs. I say nearly because is it possible for a pullet to store sperm from one of the removed cockerels for several weeks, but generally breeders observe that the more recent breedings take precedence over the older breeding, making this less and less likely as time passes.

Now, as for the 2 pullets, they both tested heterozygous. That means they will lay blue eggs (and they do), but half their offspring will inherit a gene for white eggs. This makes them less valuable as the genetic basis for future breeders, as all their offspring would need to be tested to see if they were homozygous.

In a week or 2, I will be sending off samples of other pullets to test. Once I identify 1 or more homozygous pullets, they can be paired with one of the 2 cockerels and will be the foundation of next year’s flock of all homozygous Opal Legbars.

Good breeding practices can be tedious and/or expensive, but it is necessary to advance the breeding of these exciting birds.

If you would like info about the lab that does the testing, their website is

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Isabel Crele Welsummers

My highest priority project is to add the gene for lavender into my line of Gold Welbars. I am now on the F4 generation for this project and making good progress. Egg color has been lighter than I want, but this generation has started to come close to the color of the Welbar and Lavender Marans. This pic shows the darkest egg gathered in the last 3 days from 4 different breeds, left to right:

  • Isabel Crele Welsummer pullet (Gold Welbar + the lavender gene)
  • Lavender Marans
  • Crele Welsummer (Welbar)
  • Copper Marans
Eggs of various dark egg layers