How likely for ducks to be killed?

! If you take proper precautions you can make them fairly safe. This would include a spacious, secure coop and run. Hardware cloth along the bottom two feet of the run so no predators like raccoons can grab them through it. Make it a covered run and they'll be safe from raptor attacks. Make sure to put a skirt on it so that diggers like foxes and dogs can't get in. And if you have big predators like bears in your area or just want to be extra safe you could run a few hot wires along the run. An automatic door on the coop to close them in every night even if you forget is also good. If you do these things they'll be pretty safe from danger, at least as safe as a prey animal living outside can be.

 

Just because you don't see predators doesn't mean they aren't there. A number of animals can be issues for ducks or chickens- rats, squirrels, weasels, marten, fisher, skunks, opposums, hawks, owls, foxes, coyotes, stray dogs and cats, neighbours' animals, and your own animals, as well as the rare cougar, bear, wolf, armadillo, dingo, etc.
In my area, we have skunks, rats, squirrels, fisher, marten, cats, cougars, foxes, coyotes, wolves, bears, stray animals, neighbouring animals, wolverines, hawks, owls, and the very rare raccoon.
The best way to prevent against predators is to think like them. Think, I'm hungry, where are the weak spots. Where can my teeth pull this or that out, where does my paw fit through, where can I climb to get in. Consider everything and use good quality materials. From 2ft down, I use 1/2" hardware cloth, stapled every 3 inches with heavy duty staples. From 2ft up, and even sometimes (on long stretches) 2 ft down WITH THE HARDWARE CLOTH, I use double or triple layered chicken wire, stapled every inch. On the top I get away with single layered chicken wire and around the edges of the top, overlapping types of wire. Hardware cloth is the best, but not always available or cost effective. Chicken wire is least preferable, but it works okay. You can use rabbit fencing, 2"x4" garden mesh fence, etc.
Make sure all fences are well supported both vertically and horizontally, and your coop is sturdy, secure, and ventilated. Mine can be shut in at night, but I don't like automatic doors, as if a duck is slow getting in, it's screwed. Make sure the run and coop are heavy enough they can't be flipped over (a concern with tractors- my 4 season duck tractor is on 8"x8" skids). If you can, use a covered run or overhang on your fence- coyotes can jump 4-5 ft, climb up to 9 ft fences, climb under etc. Coyotes exist pretty much everywhere.

 

What Welshies said! Ducks and chickens both are delicious and easy to catch because they are asleep at night and have poor night vision, so predators are highly motivated to get to them, and they have all night to work on getting in. Ducks are especially attractive because they don't really have any sharp parts.

I've had two losses in the 2.5 years I've had ducks. The first was the very first night - I had never seen a raccoon in the yard, but it found my sleeping duck who was right next to the wire fencing I had them in that had spaces a lot bigger than chicken wire. It reached in and ate what it could through the bars. The second was when my new neighbors' little dog found a weak spot in the cedar fence and broke through it. I once had a raccoon open a doorknob and two latches to get into my quack shack! I lock it with a deadbolt and key now. Luckily it tried to drown the duck in the pond and I was able to get her away from it. Must have thought she was a chicken?

I live in an urban/suburban area. My ducks are beloved pets and it sucks to feel worried about them. Ducks are just so defenseless. That said, I still very much prefer having ducks to chickens! They are so beautiful and their personalities are so sweet and have the sweetest little sounds as they graze, and they actually love terrible weather. Also, they're bigger than chickens so the hawks and eagles are less likely to target them than chickens. We had a Cooper's hawk hanging around last fall, that spooked the ducks but never made a move - it was stalking my neighbor's chickens. She has lost 4 chickens in the time I've had ducks, but I think one of them just got sick, and one was killed by a dog that got under her fence. Other than the freak dog thing and birds of prey, all of the predators here are pretty much a night/dawn problem, so I keep the ducks locked up until the sun is hitting the ground. I also got three of those blinking red light things that are supposed to look like a moving predator. I'm not sure if that's what is working, but we've had no sign of the raccoons in about a year. We do have possums but they don't seem to have any interest in the ducks.