Do you intend to try to breed these birds? If not, I would not keep them caged together, because this may happen again and next time she might end up dead. They are both old enough to breed, but that doesn’t mean they are compatible as a breeding pair. He may be interested in breeding and she isn’t. Typically this is the reason why this type attack will happen. Most of the time, it is because the female is too young. It can also happen the other way, where the female tries to kill a male who isn’t ready to breed. While hormones can be controlled to some degree, they will still become hormonal at times. If she is not bonded with him as a mate, then he may turn on her the next time his hormones kick in. Signs of a mate bond are sitting together, feeding and grooming each other and mating. If they just share a cage and interact here and there, they are not bonded, and this means she is in danger each time he wants to breed. There were probably warning signs such as him following her around and trying to solicit mating. It can be subtle if you don’t know what to watch for. She may be a late bloomer, or she might even be sterile. If you handle these birds, that can be a factor, because he wants to claim her as a mate, and she shows you attention. Again, I’d be inclined to keep them apart other than when they are out, or under supervision. A fatal attack can happen so quickly. You can try cages side by side, and if both birds spend their time trying to be together, that is a good sign. But if she just shows occasional interest, I would say they aren’t compatible as adults and need to stay in their own cages.
Thank you for asking Lafeber,