Hi,
If this is an occasional spat, then this is typical of budgies. If feathers are being pulled out or blood is drawn, they must be separated. If they are constantly chasing the males, or stalking them all day, this is very stressful and they should be split up. Basically, if the bullying is most of the time, then it would be best to separate them. Female budgies can be very aggressive. As for keeping a group cage, it’s best to have even numbers. With odd numbers, they tend to pair off, even in same sex pairs, and one bird gets left out and is often bullied. You should never introduce nest boxes or anything that can be used as a nest when you have more than a male & female in a cage. Nesting behavior makes the females even more aggressive and territorial and they will kill over territory, even without a mate. Budgies should never be given a nest unless it is a breeding pair.
Thank you for asking Lafeber,
Brenda