This happens sometimes with breeding cockatiels in captivity. It’s really not understood why. One reason may be if the birds are too young – especially the male. Cockatiels should be at least 2 years old before you set them up for breeding. They may be capable of breeding at a younger age, but they really are not ready. Young birds make a lot more mistakes. Young males are often much more interested in mating than settling down and caring for eggs and chicks. If your birds are not yet 2, then I would keep them separate until they are at least 2, and then try reintroducing them.
Regardless of their age, if the hen has stayed off of the eggs for hours – long enough for them to get cold, then the eggs are not going to hatch. You should remove the nest box, keep the pair separated for now, and rest them for 6 months. In the wild, they only breed once a year. In captivity, it is important to rest your pair for 6 months after each clutch. They should only be allowed to have two clutched per year. So they should be resting now anyway, and it will give the male a chance to settle down. After 6 months, you can put the pair back together and see if they will bond again. If they do, and you see signs of mating, then give the nest box back to them. But if the male becomes aggressive again, then he should not be allowed to breed again. Some birds do not make good breeders. It is very important with cockatiels that the male and female remain together for breeding and caring for the eggs and chicks. One parent sits on the eggs during the day, and the other sits at night. And both help feed the chicks. The female can’t do this by herself. So if this male is going to get aggressive each time, then he shouldn’t be used as a breeder.
Thank you for asking Lafeber,