The RSPB explanation of 'Mobbing' is:-
'Mobbing' is a noisy, obvious form of behaviour that birds engage in to defend themselves or their offspring from predators.
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. Mobbing usually starts with just one or two birds, but may attract a large number of birds, often of many species. For example, a chorus of different alarm calls coming from the same tree is often a good sign of a roosting owl or a cat.
Mobbing behaviour has been recorded in a wide range of species, but it is particularly well developed in gulls and terns, while crows are amongst the most frequent mobbers.
In addition to flying at the predator and emitting alarm calls, some birds, such as fieldfares and gulls, add to the effectiveness by defaecating 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 by a colony of fieldfares.
Mobbing behaviour has many functions. Predators often rely on surprise to succeed. As a predator has been discovered, birds will blow its cover by the loud alarm calls. This will alert other birds to the presence of a predator, and reduce its chances of success.
This noisy mobbing will also serve to impress the appearance of the predator on inexperienced individuals. The constant harassment by the mobbing birds will also drive the predator to a safe distance. The mobbing birds are seldom at risk, provided they keep the predator in sight and do not take too many chances.
A predator may be mobbed regardless of whether it is in flight, on ground or in vegetation. Birds attacking a perched or ground predator always give loud alarm calls and may make physical contact. Flying birds of prey are attacked by swooping down at a steep angle from above and behind, and emitting alarm calls. Sometimes contact is made with bill or feet.
Such attacks are rarely pressed home against really dangerous species, such as goshawks for crows. Mobbing attacks are strongest when the birds have most at stake, such as during the breeding season when young birds are at risk from a wide range of predators.
Birds may mob anything that they consider to be a danger. The cuckoo, owls and day-flying raptors are the birds most commonly mobbed, but cats, foxes, snakes, even humans, are regularly on the receiving end. The cuckoo is of course a danger species in a different way from predators, though the resemblance to a bird of prey, especially when flying, is probably a coincidence.
Since there is a learnt component to the behaviour, birds can start to mob anything they see other birds mob. Therefore, theoretically even inanimate objects such as statues or paint cans may end up being mobbed.
Mobbing is not restricted to danger species or indeed to birds of prey or mammalian predators. In this country, adult crows are likely to be taken only by the rare goshawk, but they often mob buzzards and kestrels among others.
They also mob non-predators such as grey herons, whose large size and flight silhouette they mistake for a bird of prey. In some species like crows and gulls the harassing behaviour characteristic to mobbing is also seen in other behaviours including food robbing.