It won't develop unless it's been in … When Can Roosters Fertilize Eggs. The complete egg in the shell is past the part of egg production, inside the hen, where fertilization takes place. Fertilized eggs are edible and look exactly the same as infertile eggs other than a small white bullseye in the yolk. I know that if a fertilized egg attaches it usually does so w/in the first 6-12 days. The eggs are most likely formed before the mating or is possibly fertilized by a previous copulation. First of all, it’s important to note that chickens do not get pregnant. But the completed egg is not what gets fertilized. Thus, if the hen has mated and she lays an egg, then that egg is fertilized. Each time a yolk ripens, the sperm will fertilize it provided it is in the hen’s oviduct. If the hen has not mated and she lays an egg, then that egg is unfertilized. How Does a Rooster Fertilize an Egg? That said, roosters are sort of like human males: they will probably be fertile most of their lives! If you want to hatch eggs from a specific hen and a specific rooster, you can be 100% certain of the “right” fertility by first housing the hen away from any roosters. The white is created and then lastly the shell will form around the egg. After about 10 days, start checking her eggs for fertility. Also, if there are no roosters , chickens can lay infertile eggs, suitable only for boiling and scrambling, have one and give me the other, which one do you prefer. You do need a fertile rooster to have fertile eggs. It's the yolk of the egg that gets made first. A bird’s reproductive system is very different from a mammals, specifically because they don’t have penises or vaginas. Source(s): Associate Professor of Veterinary Research. It takes about 30 hours for a hen to make one egg. Without getting too graphic, I know a lot of people are curious about how roosters mate with hens and how they fertilize eggs. This method has largely been replaced by ICSI. An egg that is not fertile will have a pale irregularly shaped spot of cells on the surface of the yolk. Roosters do help fertilize chicken eggs so that they form into baby chicks but, since they don’t have penises and chickens don’t have vaginas, at least not in the traditional sense, there is no actual ‘sex’ that is performed and no pregnancy to speak of. Veterinarians can artificially fertilize bird eggs by collecting a sperm sample and transferring the sample to the female’s cloaca. In order for it to be a fertile egg, you will need the rooster to do his part. The chance that an egg being fertilized from the semen of that days copulation is very unlikely and closer to impossible. Hens go through oviparous reproduction , which refers to the laying of eggs. And if you have a great rooster, it makes sense to allow him to fertilize eggs for as long as possible to pass on those good genes. For the eggs to be fertilized, the hen and rooster must mate first, and this process must occur prior to the formation of the egg. There are two techniques for fertilizing eggs . How do chickens fertilize eggs?