My wife had a discussion with a coworker the other day about animals. Now, I should tell you that my works in a laboratory. Birds, mammal, insects, etc. are daily discussion topics where I work but not so in a medical laboratory. My wife often fields questions from coworkers about animals, however. They think her husband knows everything about animals. This is, in fact, a misconception fueled by the fact that I do know most everything about animals. OK - the real truth is that I learn something new about animals almost everyday.
The topic with the coworker involved mobbing. 'Mobbing' is a noisy, very obvious form of behavior that birds engage in to defend themselves or their offspring from predators. A simple definition that I like is "an assemblage of individuals around a potentially dangerous predator". When a predator is discovered, the birds start to emit alarm calls and fly at the predator, diverting its attention and harassing it. Sometimes they make physical contact.
Why, asked my wife's coworker, doesn't the bigger predator just turn around and eat one or two of the smaller birds?"
Much is lacking in our understanding of mobbing. It is not clear why predators don't simply turn on their tormentors and snatch up one or two of the mobbing birds. If they did, then you would think mobbing would quickly disappear wouldn't you. Well, I'm sure it isn't that easy to spin around and grab somebody like that in mid air. Many times the bird of prey utilizes diving speed or the element of surprise to aid in the capture. Then there is that whole "which one do I choose" problem when it is a group effort. There is safety in numbers many times.
Mobbing tends to occur most intensely on the breeding grounds. For instance, in April a tape recording of the cries of an Eastern Screech-Owl brought Prothonotary Warblers, Blue-gray Gnat-catchers, and other small songbirds swarming in from their newly established territories in the swamps of South Carolina. A week later the same tape had no discernible effect on the same birds.
Mobbing behavior has been recorded in a wide range of species, but it is particularly well developed in gulls and crows are amongst the most frequent mobbers. In addition to flying at the predator and emitting alarm calls, some birds, such as those gulls that I mentioned, add to the effectiveness by defecating or even vomiting on the predator with amazing accuracy. There are reports of predators being grounded by the volume of droppings over their body after a concentrated mobbing attack!
Mobbing behavior is probably the most frequently observed overt anti-predator strategy in the bird world. It serves to alert others of a species and also other species of a predator's location. It also serves to harass the predator enough to move it along to another area.
You might even call it a form of bullying behavior.