If you have one or more fertile eggs that a hen cannot, or will not, take care of, you should try to foster them with another hen. The ideal situation is to have other hens with eggs that were laid about the same time. You can usually transfer an egg or two to a setting (or brooding) hen without too much trouble. If you don’t have a hen available, you can always try incubation as a last resort, but that’s another story.
Here’s how
to check to see if an egg is good. Warning: always handle the eggs gently, as there may be a viable chick inside. Fill a cup with warm water – not hot, but comfortably warm when you place a finger in it. Check one egg at a time by carefully putting them into the water. (I use a half-full one-cup clear glass measuring cup.) If the egg floats that indicates it has started to dry out, meaning it’s old and no good. If the egg sinks, it’s fresh and has a chance of hatching providing it’s fertile.
Determining Fertility. A few days after an egg is laid there should be red vessels visible through the egg. Hold the egg over a flashlight and peer through it. If there are red vessels, then the egg is fertile. If, after a few days the egg is still clear, then it isn’t fertile. But, remember it takes approximately three days for vessels to appear. All eggs appear clear at first. I always wait at least a week before ruling that an egg isn’t fertile. Sometimes, too, I simply leave the eggs with the hen until she decides to abandon them herself.
The photos show an egg directly on a flashlight and one balanced atop a toilet paper roll with duct tape over the top. A hole is cut in the center of the tape to craddle the egg and allow light to shine through it.
NOTE: If you have one or more babies hatch, leave all remaining eggs that didn't hatch in the nest. They help keep new chicks warm.
I’ve fostered eggs, and once even a chick. Here are my stories:
My first pair, Rhett & Scarlett, were happy together. However, of the eggs Scarlett laid, only one was ever fertile. She managed to raise a hen, Bonnie. Later, another older hen passed away with four eggs in the nest. I floated them and one sank. Scarlett successfully fostered that egg and raised the baby.
I had another hen that typically hatched four eggs, but often wouldn’t feed all of them. When her eggs began to hatch, I monitored the babies closely. As soon as it became apparent that she wouldn’t feed the last baby, I placed the new chick under Scarlett. Fortunately, she had infertile eggs that were due to hatch which I hadn’t removed. Scarlett examined the chick, but didn’t appear to harm it. Fostering chicks is very risky. Keep an eye on the situation, other hens might kill a chick that isn’t theirs. If you can, hand feeding it is safer. However, although unusual, Scarlett accepted the newly hatched chick and raised it to adulthood.
I’ve read that most hens will accept another bird’s eggs if added to their own, but never chicks. Scarlett was probably unique in that regard, but she also had a very sweet nature and that’s why I thought she might accept the chick. If I hadn’t been working full-time, I would have handfed that baby, but was unable to do so then. Scarlett and Rhett successfully raised that adopted baby.
Currently, one of my hens, Cherry, is sitting on five eggs … well, not really … she has two eggs and three babies. She had three eggs when I gave her an extra egg from a young hen who laid it on the floor of her cage. That young hen had been mating, so I believed the egg to be fertile. I could have put it in the young hen’s box, but, believing it had a better chance with Cherry, I chose to give it to her because she and her mate are excellent parents. Cherry later laid another egg, giving her five. The fostered egg hasn’t hatched so far, but I’m still hopeful.
Fostering eggs and babies is never an exact science. The young hen mentioned above has since laid three eggs in her nestbox and is sitting on them. So, in retrospect, maybe putting it in her box would have been okay.
This is Cherry’s third brood of the year and the young hen’s first. I will close Cherry’s nestbox off once her babies are fledged. Three clutches a year is enough for any bird. Too many weakens them too much and you may lose them.
I’ve been told that Budgerigar parakeets (budgies) make very good foster parents for the more expensive parakeet varieties. This is true, and I once even acquired two pairs for this purpose. They did a good job rearing young, but my birds are indoors and four budgies were extremely noisy. I gave one pair away and, when the noise level remained high, eventually divested myself of the other pair too. I’ve grown used to the soft, lilting songs of the Bourkes and Splendids. Also, I have enough pairs now that foster parents are usually available. I try to open all the nestboxes at the same time of the year. An available nestbox stimulates mating and rearing young. If all the boxes are available at the same time, most pairs will raise their young in approximately the same time period too. This helps if fostering becomes necessary.
Peace & Blessings.
Our three baby Bourkes being hand fed are now able to fly...not well maybe, but they can leave the table and end up who knows where. 
They still need to be fed and still want to hide away during the day. But, their previous shelter/box was no longer safe. It was time to move them into a cage, but still give them a place to hide. The small cage I had available has small doors and the box they were in wouldn't fit. 
