Hi,
You need to remove the new eggs and discard them. This is a first time pair, and mistakes are common. First clutches almost always fail, but the parents need to learn, so sometimes you have to learn to deal with losses. If it’s too hard on you to have chicks die, then do not breed again.
Your parents may also be too young. Cockatiels should be between 2-3 years old before pairing them with another bird. If your birds are younger than this, then they are typical young parents. They are usually more interested in mating than settling down and caring for eggs and chicks. Anytime a pair lays eggs while still caring for chicks, the new eggs need to be discarded. A pair must be rested between each clutch, even if the eggs do not hatch. Forming and laying eggs is very hard on the female, and laying while she still has chicks will take a toll on her health. The nest box needs to be removed and the pair rested for at least 6 months. Or if they are under 3, do not breed them again until they are old enough. Also, when you have a breeding pair, they need a nutritionally balanced diet like pellets – a seed diet is not adequate, especially for breeding. They also need leafy greens, veggies, some fruit and an egg food. This can be a dry commercial egg food, or cook an egg with the shell washed, crushed and cooked with the egg. When they have chicks to feed, they need a LOT of food, and it needs to be replenished throughout the day. If they do not have nutritious foods, or enough foods, the chicks will not thrive.
Most likely the little chick is stunted from not being fed enough, and/or not getting nutritious food. Such as if the parents only have seeds, the chicks will not grow or survive. It may catch up some with handfeeding, or it may still die. Unfortunately, the parents didn’t learn – I do understand why you pulled it – but if you try to breed them again, they may make the same mistakes. And some birds are simply not good breeders. I would recommend taking the chick to an Avian Vet so they can at least check for organisms that might be affecting the ability to grown and develop.
All of this can happen with parents that are old enough and on a perfect diet. Again, first clutches almost always fail. But you may also consider that there could be a health issue with one or both parents. Lots to think about – that’s why I always tell owners that breeding birds is difficult and can be very expensive. I hope the little one makes it, but you did your best and again, there could be health or genetic factors you don’t know about.
Thank you for asking Lafeber,
Brenda