I think you're looking at this scenario all wrong. I strongly believe that you do not have just 1 culpret.
Sounds like you have a whole bunch of different critters coming in close now that it is winter. This happens alot to livestock.
You can go all summer with no problems because there is food in abundance. But as soon as it gets cold out, those predators look for easy meals.
If you find a dead duck with the head missing, it's allmost allways the result of a bird of prey. Like your owl friend there. They will eat the most nutrient part of the animal first, the eyes and brains. They do that because they dont need to expend much energy to eat that part. Then if they get desperate, they will come back for the rest.
Racoons and minks and weasels will also be a problem. But they usually just kill the bird, and eat it right there on the spot. One time when we were younger, we found 3 headless birds IN the chicken coupe. Turned out that it was just racoons. We also had problems with rats. They would litterally eat the feet off of our birds WHILE they were alive! lol....
That didn't last long, but it sure was a shock.
I think the best thing you can do is just put up some better defenses. The net over the top of the pen is a great start, plus locking them up at night will help as well. You can also try finer mesh wire along the bottom couple of feet, or bury the first few inches, that will help keep some things out.
Oh yeah, and don't even consider killing an owl.... a dead owl found on your place can cost you an arm and a leg. but if you DO need to remove it for reasons of depredation, then call the WDFW. But, I can tell you from first hand account that unless those few ducks are your only means of food or income, the state won't really do a thing. The nuisance animal pretty much has to kill off a cetrtain amount of moneys worth of livestock for the big guns to be called in.
I wish I could see your coop, maybe then I could help you better with advice to help protect your pets.
Good luck with the varmints.
