A flock of furious crows invaded the territory of one of the hospitals in St. Petersburg.
The crows attack all the people who come to “their territory”, including patients and doctors. Defenceless doctors do not know what to do.
In order to protect the patients from the bloodthirsty birds, the administration of the hospital put a warning sign on the entrance door:
“Danger! Man-eating Ravens!”
“We are in despair! They consider themselves to be the full-fledged owners of this territory. When the crows come to our area pigeons, cats and even stray dogs disappeared.
Our defenceless workers are afraid of going out. One of our nurses has already fallen the victim of these monsters. The whole flock attacked her and injured her severely,” says the chief doctor of the hospital Fedir Baranovski
Birds that breed in colonies such as gulls are widely seen to attack intruders, including encroaching humans. Behavior includes flying about the intruder, dive bombing, loud squawking and defecating on the predator.
Costs of mobbing behavior include the risk of engaging with predators, as well as energy expended in the process.Black-headed gulls are one species which aggressively engages intruding predators, such as Carrion crows.
Classic experiments on this species by Hans Kruuk involved placing hen eggs at intervals from a nesting colony, and recording the percentage of successful predation events as well as the probability of the crow being subjected to mobbing.
The results showed decreasing mobbing with increased distance from the nest, which was correlated with increased predation success.
Mobbing may function by reducing the predator’s ability to locate nests, in other words as a distraction, since predators cannot focus on locating eggs while they are under direct attack.
Adaptationist hypotheses regarding why an organism should engage in such risky behavior have been suggested by Eberhard Curio, including advertising their physical fitness and hence uncatchability (much like stotting behavior in gazelles).
Distracting predators from finding their offspring, warning their offspring, luring the predator away, allowing offspring to learn to recognize the predator species, directly injuring the predator or attracting a predator of the predator itself.
The much lower frequency of attacks between nesting seasons suggests such behavior may have evolved due to its benefit for the mobber’s young.
Niko Tinbergen argued that the mobbing was a source of confusion to gull chick predators, distracting them from searching for prey .
Indeed, an intruding carrion crow can only avoid incoming attacks by facing its attackers, which prevents it from locating its target.
Besides experimental research, the comparative method can also be employed to investigate hypotheses such as those given by Curio above.
For example, closely related species such as the Kittiwake do not show mobbing behavior. The kittiwake’s cliff nests are almost completely inaccessible to possible predators due to gusty winds and the sheer cliffs they nest in, meaning its young are not at risk to predation like the Black-headed Gull.
This is an example of an evolutionary pattern known as divergent evolution.
Mobbing is thought to carry risks to roosting predators, including suffering harm from the mobbing birds or the risk of attracting larger, more dangerous predators.
Birds at risk of mobbing such as owls have adapted cryptic plumage and hidden roosting sites in order to reduce this danger.