Two images of a Cooper's Hawk harassing the female Great Horned Owl.
Top image: The female Cooper's (top right) is giving the Owl (bottom left) a mouthful. She kept this up for quite some time, hopping from branch to branch. At times she got within several meters of the Owl. All this time, the owl simply kept an eye on the Hawk, never moving from its perch.
The Cooper's, now above the owl keeps on squawking loudly, then realizing the owl is not moving, it finally flies off. All this time the male GH Owl remained perched in a nearby tree ready to pounch if need be.
This scenario is often repeated, not just with Cooper's Hawks, but more often with Crows and also at times with Sharp-shinned Hawks taking turns to harass the Owls. I think the harassers are trying to lure the Owls into a chase so they can raid the nest; however, the Owls aren't taking the bait.
