Of course i realize that the ducks on the pond face more danger from predators than ducks locked up in a pen. Some years back i had at least 150 ducks and geese on a 50-acre lake that a bald eagle apparently considered his pantry. And the eagle attacked in the daytime. So i have had some experience with a number of predators but never turtles before, at least to my knowledge.
I have spent a lot of time watching these ducks, and they seem very vigilant about what is going on around them. They react to any slight change such as a shadow flying over so quickly that i doubt a goose would have the opportunity to honk a warning before the ducks had realized the danger and already reacted. This attack from below by a turtle is new to me and i assume would be hard for the ducks to anticipate.
It is hard to lose a bird, but it is a wonderful thing at least to me to see the absolute joy these birds exhibit doing whatever they want to do, as you put it. Splashing, vigorous tail-wagging, swimming underwater and popping up fifteen feet from where they submerged, the usual upside-down fishing, wings flapping beating the water.... They eat some of the corn we put out for them, then rush off in a group to harvest some sort of bug, i suppose. Back for more corn. A swim and splash. Digging in the mud. Busy, busy, busy. Maybe it is irresponsible to give them the freedom, but i would hate to take away their joy. I have done it both ways over the years as far as confined and not confined, and the life of a duck is vastly different in both scenarios.
In the meantime, we will be doing what we can about the turtles and hope they will start using the raft and house we built for them.