For my entire career as a forester (over 20 years now), I have noticed small circles of downy feathers on the ground. These circles are usually no more than a foot in diameter and comprised of only the finest of feathers–no bones or skin as you would find from the remnants of a kill.
One morning a while back I was walking down a woods road, heading to the starting point for a timber inventory I was conducting. Suddenly a loud whoosh startled me, followed by a plumed explosion 20 feet in front of me. A hawk had zipped by my head and snatched a songbird off of a branch, hitting it with enough impact to cause an eruption of downy feathers. Sure enough, the cloud hung in the air briefly, then settled on the ground in a perfect circle. Mystery solved.
More on the Cooper’s Hawk: https://en.wikipedia.org/wiki/Cooper%27s_hawk