Possum will kill chickens. They really like eggs too. Possum are pretty slow but they climb really well and can catch chickens when they are on the roosts. During the day they are not that big of an issue. They are mostly nocturnal but I’ve seen a possum feeding from my compost pile at 1:00 pm on a bright sunshiny day. Mostly nocturnal does not mean 100%.
If you have a live trap, or can get one the right size, they are not that hard to trap. I often wad a spoonful of peanut butter in a paper towel and toss that in the back of the trap. But you also might catch a raccoon or skunk doing that, though other baits usually work better for skunk. You have to come up with a way to dispose of one if you trap it too. It’s generally illegal to release one anywhere without the landowner’s permission. You can kill hem but depending on where you are there may be legal restrictions on that. Those methods are fairly unpleasant too and you need to dispose of the body.
There is another aspect to it also. Just because you get one doesn’t mean you have all of them. Since the first of this year I’ve permanently removed one raccoon, six possum, seven skunks, and eight rats from my property, three acres in the country. One skunk went into my garage through a doggy door, sprayed, and left. I don’t know if I got the right one or not. Where there is one there will be more. Most of these were caught in a live trap but I also have a shotgun that comes in really handy at times.
I’m all in favor of permanently removing any unpleasant critter that is hunting your territory, it reduces your risks. But the only way to really protect your chickens is with good barriers like fences.
I have not tried that ammonia rag trick. In my mind that’s only going to be temporary if it works at all. I’ve tried various things like Irish Spring soap, dog hair, coyote urine, and human urine with little success. I find that wild critters are very adaptable and soon realize there really isn’t any real threat to them. Good luck with that but I’d get to work on some good fences or other barriers they can’t climb over or tear through.