I have two 4.5 month old mallards. I just got two welsh harlequins that are now two weeks old. The mallards are very agitated by their presence and to make matters worse (or at least not better) they have begun trying to mate. We have the ducklings in a pen that the mallards can see but not get into. I'm sure if they were loose they'd be getting attacked. How long will this behavior go on and when should I start letting them be together? Do we have to wait until the ducklings are the same size?