I dont have time right now to read this entire thread, but, do want to comment...
Last year we were soooooo excited when we brought our first 4 Ducks home! It was in August..
They were Free Rangers in our backyard, eating bugs, sleeping and hanging out wherever they wanted.. it was great..
Until about 5 days after we got them, my husband came home from working a night shift to find they had been attacked! We realized from the wounds, combined with what we had seen in our area that it was a Red Tailed Hawk that got them. 2 were dead (killed from attacks on the top of their necks---sure sign of a hawk attack). Another had a wing that was ripped to shreds, and one was unharmed.
We buried the 2, stowed the unharmed one in a playhouse for the time being, and took the one with the broken and torn up wing to the vet. She got an amputation and made a full recovery.
We now keep those 2, along with 3 others (and now 2 ducklings) in a pen ALL THE TIME. After the attack it took us a mere 2 days to construct our pen, we worked day and well into the night until it was done.
It is now 9 months later and we have not had a single attack since building the pen!
So, YES Birds of prey will KILL full grown ducks with no remorse.