On a chilly winter day a flock of crows has discovered an owl in a tree. Like an angry mob, the crows surround the owl, haranguing it with raucous cries. This behavior is called mobbing. It's a kind of social attack that is common in many different kinds of birds. Generally, it's small birds that attack larger, predatory birds, but birds will also mob terrestrial predators, like housecats, and sometimes even humans.
Some birds are much more likely to mob a predator during nesting season than at other times of the year. But crows like these will almost always mob an owl on sight. Mobbing is generally just persistent harassment. But sometimes smaller birds will physically attack an interloping predator and may even successfully kill it.
Mobbing is most often seen when birds are in flight. A passing eagle or hawk will be intercepted by a single bird that repeatedly dives at it, harassing it as it flies. A flock of birds may also intercept a passing predator. An eagle scoping out a flock of gulls on a beach may find the tables turned when instead of fleeing, the gulls engage it and drive it away.