UPDATE!!! We brought the ducklings back to the pond and found mama duck!!! She came waddling over with her two remaining babies and the three rescues bounded out of the box to meet her. It was so cute. The mother started preening her babies and quacking all worried and checking them over and then the family headed back out to the water. It made me so happy seeing them all together again! I'll find the pics I took and post them in a bit.
@FunnyfarmMidwest yes, she looked like a pure mallard, but the father could have been anyone. There's like a million different breeds there, Runners, Cayugas, Calls, lots of Pekins and there's a flock of wild wood ducks there now too.