Adding to the flock?
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I personally suggest having a few weeks of separation where they can see each other but not reach each other.
- Overrun with Runners
I think waiting till they are closer to the same size before they can touch each other - but frequent time near each other on opposite sides of a fence is the approach I would take.
Also, I would have fecal tests done on the new ducks to be sure they don't have parasites. Better to treat a couple of new ones than have to treat the flock.