Cooper's hawks will definitely kill full grown hens. They're a primary threat around here. We have a lot of cover, and they excel at hunting through it.I didn't expect hawk attacks to grown hens... Especially with tons of smaller prey available such as squirrels songbirds rabbits and other rodents. There hasn't been any trouble in months!
Since you have a nesting pair nearby a secure covered run for your chickens is probably a good idea for now.