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. Another reason he might be doing this is because he wants her to be in the nest box. However, even when a pair is using the nest box, some males behave this way. Another issue might be that they are hand fed and tame. Breeder birds should not be handled – at least not when you have them set up for breeding. The male may feel torn between his mate and a human he likes. Or he may be jealous of a human she likes, so he might attack the hen if he can’t reach the person he either likes or is jealous of.
Regardless of their age, the hen can’t do this alone. With a pair that is using the nest box, one parent sits on the eggs during the day and the other sits at night. It’s too much work for one bird – the hen will get run down physically if she tries to sit on these eggs around the clock. Most likely she will not stay on them at night. Also, the cage floor is not ideal – if any chicks hatch, it’s hard for the parents to keep them all together and to keep them warm enough. Is there a metal grate on the cage floor? If so, the eggs won’t develop anyway because they won’t stay warm enough. If the pair were getting along, I would recommend moving the eggs to the nest box to encourage the parents to go inside. Is the nest box attached to the outside of the cage as high as possible? Do you have nesting material in it, like shavings? Most cockatiel do not want nesting material, so if you added any, she may not like that. In this situation, you can try rotating the male and female to let them take turns sitting. Sometimes after a few days of this, the male settles back down. But if this doesn’t work, and she doesn’t sit on the eggs all of the time, I would discard the eggs. 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 clutches per year. So they should be rested if these eggs are not incubated or fail to hatch, 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. If this male is going to get aggressive each time, then he shouldn’t be used as a breeder.
Thank you for asking Lafeber,