Hi,
The first thing you need to do is split them back up! Both birds are too young to be with a mate yet. Cockatiels should be at least two years old before you pair them up and try to breed them. While they are capable of breeding at a younger age, it isn’t natural and would not happen in the wild. Young birds are still developing physically and emotionally for at least two and up to three years old. For a young female, she is at a high risk of becoming egg bound and dying. Her reproductive tract is not ready to process and lay eggs yet. Because it is still developing, the egg tends to get stuck because she simply doesn’t have the developed muscles to lay the egg. Young males are typically more interested in mating than caring for eggs or chicks. They are very likely to develop bad habits that will ruin him as a breeder. He may start breaking or even eating the eggs. The female may eat the eggs because she knows she needs the calcium and protein. Once a bird starts breaking or eating eggs, they will never stop. Young males will also refuse to help incubate the eggs, or he might help at first, and then want to mate instead. He can get aggressive towards the hen and she can get aggressive towards him. When birds get in the habit of not sitting on eggs or abandoning the clutch too soon, again this is a habit that usually can’t be broken. Give these birds time to finish growing up and maturing, each in their own cages. When both birds are over 2 years old, they are much more likely to bond as a pair. Right now, she has no use for a young male because she knows he is too young to be a reliable mate. And he will get more and more aggressive as long as she keeps rejecting him. Instead of having a good breeding pair, these two are destined to not bond or even like each other if you do not separate them until they are old enough.
One more thing – hopefully these birds did not come from the same breeder? If there is any chance they are related, then you can’t allow them to breed. If you are concerned they are related, you might see about trading one to someone else for an unrelated bird.
Thank you for asking Lafeber,
Brenda