I have not read anyone else opinion on this matter, but as an local bird rescue person, please let me assure you that it is acceptable to have wild bird as long as you don't prevent them from leaving. IN fact, AND THIS IS IMPORTANT, wild birds become very dependant upon help offered to them. If you just haul these birds off, they have no training for the wild, and no flock to join, and nobody to help them, and they will likely die, or end up with a rehabilitator after they are found nearly dead from lack of food. It is important that truly wild birds, like these ducks, be free to leave. Don't pen them in anything less than 12 x 12 (ducks can and will escape that if they want) and NO TOP for the pen. Do let them have more space if you can. They may choose to follow a flock this fall, and thats ok. DON"T PREVENT them from leaving in any way, but don't just abandon them now that they depend on you for food. My hands are full with many such other poor birds who some unknowledgable person freed, thinking to do them a favor. It was no favor, believe me.