You may have seen a group of crows or other birds appearing to attack a hawk or owl sitting in a tree. This is called mobbing and is done to a number of predators, especially ones that prey on young birds still in the nest, such as snakes, fox and cats. Crows, red winged blackbirds, grackles, mocking birds, blue jays, chickadees, all will instinctively gang together and pester predators. It is more common with species that hang together in flocks. The harassment is very vocal, and many birds have a specific mobbing call so that other birds know what’s up and come to help out. The mobbing call is usually loud and boisterous, and easily heard from a distance. Other behaviors often seen include circling the predator, dive bombing, and even well aimed defecation.
Crows and blackbirds usually attack as a single species, but smaller birds will sometimes group up with other species. The mobbing birds rarely actually attack, but merely try to make the predator uncomfortable enough to leave. And when it does the mobbing birds pursue it with even louder calling.
Birds are by far the largest animal group that uses the mobbing technique, but there are a few others. One species of ground squirrel will harass snakes close to their nest, and blue gill have been observed mobbing snapping turtles that come near their colonial nests.
The reason for predator harassment is simple. Mobbing isolates and identifies a predator with little risk to the individual bird. It usually results in the predator leaving the area, or at the least makes using hunting stealth impossible. Predator detection is benefits the participants as well and other prey animals nearby. Harassment can drive off a predator to defend nesting sites or prime feeding areas, but is also used as an offensive tactic to obtain food by driving a large predator or scavenger away from a kill.