The location of a roosting owl is frequently given away by the combined alarm calls of a flock of smaller birds. An Eastern Screech-Owl was the object of attention for this group of about twenty birds that included Dark-eyed Juncos, Carolina Chickadees, Tufted Titmouse, Carolina Wrens, Northern Cardinals and both nuthatches (Nov, PA).
Below the owl were other tell-tale signs of its presence: pellets on the ground and whitewash on lower branches.
Owls are not the only victims of mobbing; in this case it was a perched Cooper's Hawk. Birds here include Warbling Vireo, Eastern Kingbird, Blue-gray Gnatcatcher, Gray Catbird and Baltimore Oriole (June, NJ).
Mobbing often starts with one or two birds, with more arriving in response to the commotion. The main function appears to be to expose the location of a hidden predator. Other common mobbing targets include snakes, cats, and predatory birds in flight.