Hi Elle,
I’m very sorry for your loss. If one bird was always picking on the other and fighting with it, then this is very possibly why the bird died. That type of treatment is very stressful, especially for a young bird. Even if he wasn’t causing obvious injuries, the bully bird could have been injuring the other bird internally. If you only had one food and water dish, the bully may have prevented the other bird from eating and drinking. Some sparring and harmless fighting is normal, but if a bird is relentlessly bullying another, the birds need to be separated. It’s too stressful and not fair to the bullied bird. It could be that the bird was weak in some way and that is why he was picked on. And there could be other reasons why he died. But I would not put another bird with the remaining one right away, and if you do get another bird, do not put a baby in with him. Always observe birds when first caging them together, and separate them if they fight a lot or if one is being a bully.
Thank you for asking Lafeber,
Brenda