Why Do Some Eggs Have Two Yolks?
A common misconception is that a double-yolk comes from a “twin egg.” It may be possible for a fertilized double-yolk egg to produce twin chicks, but the eggs we purchase at the grocery store are not fertilized.
In a fertilized egg, the yolk provides a source of food for the developing embryo. As the chick matures, it pulls nutrients from the yolk until it is strong enough to break through and join its peeps on the farm.
While there are some breeds of hens capable of producing double yolks, most double-yolked eggs we discover inside the cartons on our refrigerator shelves are simply biological practice swings.
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.
The majority of these double-yolk eggs are produced when a hen is still quite young and her body is adapting to egg-laying. 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, and are typically longer and larger than a single-yolked egg. Once in a while, hens may even produce an egg with no yolk at all. These rare eggs are known as “wind eggs” and usually occur during a hen’s first egg-laying effort.