Well, a hen’s reproductive system consists of an ovary and an oviduct. The ovary contains undeveloped egg yolks (the number of yolks (or ovum) that are contained here are the total number of eggs the chicken will lay in her life) that are released into the oviduct as each yolk develops, usually about an hour after the previous egg was laid. However, in young pullets and some heavy breed hens, two yolks are sometimes released within a couple of hours, and these become double-yolked eggs.
Heredity can cause some hens or breeds to have a higher propensity for double yolks; but it most often occurs in pullets that are just beginning to lay. It sometimes takes a bit for their systems to “get-in-the-groove” of egg laying. Typically, as hens mature, their systems settle down and they’ll produce one single-yolk egg approximately every 25 hours.
Double-yolked eggs are safe to eat (yes, I’ve been asked this), and are typically longer and larger than a single-yolked egg. The darker brown egg in the photo is actually a jumbo-sized (2 ½ oz.) egg, whereas the lighter brown eggs are 3 1/8 oz. and 3 ¼ oz.; and were both double-yolkers.
Each time we’ve raised young pullets, we’ve seen a few double-yolked eggs; and we’re seeing quite a few now that the Buckeyes are laying. As the Golden Buffs, Barred Rocks, and Red Comets got older, they quit producing any double-yolked eggs – it’ll be interesting to find out whether the Buckeyes quit as they get older too, or whether they’ll continue to lay a few (Buckeyes are considered a heavy breed hen).
Double-yolk eggs are not really that rare (about 1 in every 1000); however, they’re rarely seen today because commercial operations candle the eggs, separate out the double-yolkers, and sell them to make egg containing products. I can remember when a double-yolked egg would occasionally turn up in store bought eggs; but today they’re typically only found in backyard or farmstead raised eggs. It’s too bad we see them so rarely; I remember being told they brought good luck. At the very least, they’re a great curiosity for kids.