I have made a similar cross with Black Copper Marans and Cream Legbars rather than Cuckoo Marans and Cream Legbars. Below in the mother (BCM), father (CLB) and offspring (Cuckoo Cross Breed).
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1) First question: What is the make up of your silver crested Cream Legar? The reason I ask is this is an English Breed and the Poultry Club of Great Britain has a standard for the Silver Legbar and they have a Standard for the Cream or Crested Legbar and genetically they are different. There however is NOT a standard for a Silver Crested Cream Legbar so I am not sure what its genetic make up would be (which could play into your question on genetics).
2) I read an article on the Opal Legbar Project in this quarter's Newsletter from the Cream Legbar club. The Article started by clarifying terms that would used in the project. It had a really good explanation of what Auto-sexing is and how it is different from a Sex-link and why that is important when working on projects that involve a cross of breeds. So with this great explanation so fresh I am eager to clarify what these are since they matter in making crosses. A sex-link if a cross of two breeds that can be color sexed as day old chicks. You how ever can not breed sex-links together to get more sex links. The only way to get more sex links is to again cross the two breeds to make the cross a new. Auto-sexing birds can be breed together and have offspring that can be color sexed as day old chicks. (Note: the Opal Legbar project had to keep very careful records of all their birds to they would know what they could breed together to more the project forward and what would ruin the project by making the wrong crosses. So Crossing a Black colored cockerel to Cuckoo hens will not make an autosexing bird but rather a sex-link.
The problem with cuckoo colored birds and black roosters in that they both are build on the extended Black e-loci primary color pattern. The sex-linked barring does not show enough distinction between the two genders on the black chicks down. The sex-link works because only the males get the sex linked barring. The females don't get the barring at all so you have males with white spots on the head and females with no spot. With the Cuckoo Marans cross Cream Legbar all the chicks will get the black chick color from the Cuckoo Marans side and all the chicks will be barred. Both males and females will have the white headspot so they will NOT be autosexing. Trying to sex them will be the same as trying to sex pure cuckoo marans. You could guess and if you do a good job get 80-85% correct but you would not be 100% correct and to qualify as autosexing you have to be able to get 100% accuracy with the color sexing.
When you move from the black chick down to the chipmunk pattern down things change. The sex linked barring is on W chromosome but not the X chromosome. Males have two W chromosomes and females and one W chromosome and one X chromosome. So the Males will carry two sex-linked barring genes and the females will only carry one (That is assuming the males are pure bred and not a sex-link. The sex-link only carry a barring gene from the W chromosome they receive from the Cuckoo mother but not from the W chromosome they get from the back cockerel). While the double diluting of the barring gene does not have any effect on black pigments in the chick down it does have an effect on the red pigments in the chick down that make up the brown colors of the chipmunk patterns. So you have male in autosexing breeds being diluted twice as much as the females in the breed. The distinction is clear. The males end up a lighter color than the females and have diluted stripes. The females with have distinct stripes and darker down color. The distinction is 100%. I have hatched 100's of Legbars and never got a single one sexed incorrectly.
Note: The Legbar x Bielefelders works because they both have the chipmonk pattern down at hatch.
Marans and Legbars do make a nice olive egg though. Here are some egg from the barred legbar x marans in the photo above with a reference marans and reference legbar egg for comparison.
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