Your birds are much too young to breed and need to be separated right away. Cockatiels should not be allowed to breed or be set up in pairs until both birds are 2 years old or older. They should definitely not have a nesting box at only 8 months old. Please separate these birds and do not put them back together until they are 2 years old. Do not give them a nest box until then, either. It sounds like you need to do some research on breeding cockatiels. If you let young birds breed, the hen is at a very high risk of becoming egg bound and dying. The male is not likely to want to settle down and help incubate eggs if she is able to lay eggs. At this age, it is unlikely that either bird would sit on the eggs. They are both likely to develop bad breeding habits like not sitting on the eggs, breaking the eggs, eating the eggs, not caring for chicks, or even killing the chicks. Both of your birds need more time to finish developing physically and emotionally. If you wait until they are 2, they are much more likely to be a successful breeding pair. If you keep them together now, and the hen doesn’t die from egg binding, the birds will probably not be successful and are more likely to never be good for breeding. It’s better to be patient now, and have more chance of success later when the birds are old enough. You have to be patient and responsible to breed birds, and that means not letting birds this age breed.
Thank you for asking Lafeber,