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- Overrun with Runners
You are right about what is going on, I think.
So. Ducklings do need company . . . can you set it up so that the one older duckling is in the same brooder but separated by plastic poultry fence, with its own water and food? That way it can see the others and not harm them.
As darling as ducklings are, they'll go after smaller ducklings very often.
An unbreakable mirror is a big help for lonely ducklings.