Hi Julie,
Do you have any nests or nest boxes in the cage? Or bird huts, tents, etc? If so, you should remove those. Budgies typically get along well in a group as long as there is an even number of birds, and no nests or houses of any type are in the cage. Once breeding comes into the picture, aggression and fighting over nesting sites will begin. That’s why you can only have one pair of birds in a cage for breeding. You should also have as many feeding stations as birds in the cage, to keep one bird from preventing others from eating. Is the bully bird a female by any chance? If so, this isn’t surprising as they can get very aggressive. If there is a big age difference, this can cause aggression. Even if you don’t have nests, some degree of mating behavior and bonding will happen, and a mature bird tends to have no use for a young bird and will attack the young birds. Did the bully bird like sitting in the food bowl, like she was using it as a nest? If so, use smaller cups. There’s a reason why she or he was attacking – jealousy over another bird, or a toy or the food. Budgies are very social, so keep one alone is not ideal. Did this bird seem to be close to the one it wasn’t attacking? If so, maybe those two can live together. If you had no nests, plenty of food stations and the bully bird wasn’t trying to nest anywhere, then I can’t recommend putting it back with the group, again especially if it is a female. But if you did have nests and just the one food area, then once you remove them and add more food cups, you might try putting the bird back, under very close supervision. If it immediately starts being a bully again, then it needs to be removed again.
Thank you for asking Lafeber,
Brenda