When you are a small bird, the last thing you want to do is fight a huge predatory hawk.
In nature this seems to happen quite often. The trick is that the birds will “fight” in large groups in a behavior termed “Mobbing“. This mobbing behavior includes actions such as emitting alarm calls, flying at the predator, often making physical contact with the predator. Some species of fieldfares and gulls will attempt to defecate and or vomit on the predatory bird. This mobbing does not always comprise of one species, multiple species can be seen in the mobbing of a single predator. Mobbing can be seen in a variety of avian species, but is most commonly seen in fieldfares, gulls, and crows. The picture to the left is a mob of crows harassing an eagle. Mobbing is also done on terrestrial predators such as cats, really just anything that can be perceived as a threat, even humans.
Why would they do this? From an evolutionary standpoint, because it works.
The mobbing behavior acts to distract predators and to alert others to the presence of the predator. This prevents the predator from surprising any individual making the likelihood of predating on anything very slim. It seems that the mobbers are not in anymore danger either as they are too aware of the predator, but it is very energetically costly to mob all perceived predators. The benefits of the behavior must out way the energetic cost if it is to continue being expressed. The greatest benefit can be seen during breeding seasons close to breeding grounds where offspring are especially vulnerable to predation. During breeding season and near breeding grounds the mobbing is much more intense then in other areas at different times. The intensity and timing of mobbing behavior can vary from species to species. One study suggests and confirms that some species mobbing is related to sexual selection. Mobbing stops once the predator is at a safe distance away from the nests or breeding grounds.
Mobbing is confirmed to be a learned behavior. Young fledglings learn what to mob based on what they see other birds mob. If the naive individual is near a population that mobs certain individuals, the naive individual will grow up and mob at the predators it saw being mobbed. This mobbing is suspected to also teach the young offspring what is dangerous. In one study an individual was taught to mob a plastic bottle. In experiments studying the phenomenon, they use a fake model of a predator and tapes to mimic the presence of a predator.
There are other ways mobbing techniques are used such as in food stealing.
Click to see GIF.