babies thrown from the nest by older sibling

Question
hi
a few days I found 5 newly hatched baby budgies outside the box, 2 youngest dead. I put them in the box and a few hours later they were thrown out again, cold but alive.
I got shocked as  never seen it before and the mother successfully raised 5 babies a few months ago.
I put them back with the mother and she covered them and all is fine. The father came back and fed them so it even more made me confused.
Then babies started to make noise again and their daughter for the previous clutch (now 3 months old)went in the box and picked up one of the baby and tried to throw it out before I stopped her. The parents did not do anything. I caught her and put in the separate cage.
Now a few days later, the babies are thriving and the other 4 babies from the previous clutch (2 males and 2 females) are not repeating that behavior. also the other 3 adult males and one female also don't go in the box.
I have never seen this behavior before
why this young budgy is behaving this way?
Val

Answer
Val,

I have no idea why bird will discard their own young. It is quite common, could be for a number of reasons. As for the older siblings throwing them out of the box, I have learned that budgies can be flat out mean to their own kind. As far as I know there has been no research or studies as to why budgies act that way. I would say that you learned a valuable lesson, I always try to keep newborns and their parents seperate from other birds while they are rearing their babies. they do not have to compete for food or nest box space when they are the only ones in there. Best of luck!