I would say a bird of pray. Look up. Are the feathers where something sat up in a tree and ate? Large birds such as hawks and owls can eat bones and all. We have found only legs left from falcons. We lost 42 (small) chickens one year to birds of prey. We hardly ever saw them. Crows eventually tattled on them and we witnessed a falcon trying to fly off with a chicken nearly the size of itself.
We can't shoot hawks in Kansas but shooting at them does detour them. They are very smart. We had one that would see me go for the gun... and wait 'till I came back with it to leave.
The circle comes from the meal they have quick. Picking out the downy breast feathers to eat a hunk of meat fast to lighten the load before flying up in a tree somewhere else to finish wings and tails.
Things that help -
Walmart Mylar pin-wheels.
Walmart has pinwheels out in spring. Get the silver and red metallic ones. The flashing messes with the bird. It can't figure depth perception on the flash and thinks it might get hit. The red also mimics predator eyes. Mount them to fence posts with zip ties or stick them in the ground. (your chickens won't care - they aren't predators.)
Multiple roosters/gobblers - Birds of prey are territorial and make calls to claim their space. Enough birds crowing/gobbling in your fields make it sound over populated and less chance at being sneaky.
Larger Animals - We did dogs and goats but lost mutiple birds training dogs not to play with them. Goats, donkeys, llamas -... ect. Things that are chicken safe and big will detour them.
I do run turkeys with my chickens and have for years. I only had 1 bird that ever got the blackhead. I treated him with wormer and he was fine.
We NEVER leave birds out after twilight. You can NOT owl proof a pasture. They swoop down on our vehicles when we come in late nights. Our birds are put up a half hour before sunset. They are fed once in the evening. They come in for it unless they are overfed. We use a border collie to round up the occasional straggler. Some chickens seem to be a bit sentimental about watching the sun set... but I have work to do.
Scare crows will not help. Not even if they are animated. (Don't ask).
That's my experience on the subject - Hope it helps!