Has OP tried to contact a licensed Water fowl rehabber? they may take the ducklings and would know the correct time to release then.

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Has OP tried to contact a licensed Water fowl rehabber? they may take the ducklings and would know the correct time to release then.
Can you keep them till later in summer when they begin to fly, they may just leave on their own. Then you will have given them time to fully become adults.
Part of the sketchy situation with humans getting involved in wildlife situations is raising them takes the fear out. That fear is oh so often necessary for survival.
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They can honestly advise you based on relative experience where to go from here, how to prepare for release, if they suggest release and where, and how, or where to get them from here to prepare them for that.