Yesterday, we saw (most likely) an owl flying through the woods at our property on the shore in Maine. The owl was being mobbed by crows and was a very large, brownish bird, most likely a great-horned owl. It flew through the woods, took another stand in a tree, but the crows were relentless. This article on Owl Spotting indicates that one way to spot a Great Horned Owl is when crows are mobbing the owl. It also says that the owls may leave a whitewash on the branches and trunks of the trees. Here is an interesting thread of discussion regarding whether the mobber or mobbee end up as a fatality.