What Defines A Marans?
The defining characteristic of this breed is the large, chocolate brown eggs they lay. The brown color in Marans' eggs is the result of a layer of pigment deposited over a finished egg as it passes through the oviduct. This is different from other "grocery" brown eggs, which are much lighter and chalkier in color, and in which the tan pigment is built into the shell calcium. In the case of a "grocery" brown egg, the color can’t be diminished by washing with water, whereas with a Marans egg, the coating may be scrubbed off using water. The coating on a Marans egg may be smooth color, stippled, or even with larger spots, because it is the result of how the layer of pigment is deposited by glandular secretions as the egg moves through the oviduct.
The genetics of what makes the chocolate brown egg are not well understood at all. It is thought that several genes may be involved, all playing some role in the final outcome of egg color. Breeding for the darkest coating on the eggs is desirable, but difficult, since the genetics are basically unknown. The best results are obtained by breeding pairs from the darkest eggs, and who lay the darkest eggs for the longest period.
Understanding The Dark Egg Properly, In Context
Sometimes a new pullet who is just starting to lay will have an egg "stall" going down her oviduct, giving the egg an abnormally long time to get coated, making for an abnormally dark egg. But further into her laying career, she is never able to produce these eggs again. Therefore, it is important to understand the basic method of rating egg color in a Marans layer. To even qualify as a Marans specimen, a layer must be able to produce a #4 or darker egg reliably for a period of her laying season. The French do not even grade egg color on newly laying pullets until they have produced at least a dozen eggs, then they rate the eggs that come AFTER the first dozen. This helps to disqualify the pullet whose first eggs may be slow through the oviduct, thus resulting in abnormally dark color that later in her life is never reproduced again.
What You See in Pictures is Not Really What You Get, All The Time
The ability to lay dark eggs waxes and wanes. The pictures you see of beautiful dark eggs are the best of what the hens lay, and no hen will lay such eggs every time. While some breeders will claim to know, the truth is that no scientific proof is yet recorded that proves that egg color is related to diet, hormones, age, light exposure, etc. The only thing we KNOW is that it is somehow, complexly, related to genetics. And we do know that throughout a hen's laying cycle, the first eggs are typically the best, and she may then go on to lay eggs of lighter color, which can then darken again after a laying rest, or after a period of broodiness, etc. If you think of her oviduct as a spray-paint tunnel, then conditions that slow down the speed of each egg through the tunnel, or cause fewer eggs to pass through the tunnel, will enable more "painting" on the egg, and therefore a better, darker color. Conditions that cause lots of "painting" (such as many eggs in a row with no days to re-build pigment supplies) will use up the paint reserve, leaving only what can be generated on the spot to "paint" onto the eggs.