If you’ve ever seen a group of small birds frantically calling, diving, and harassing a much larger predator, you’ve witnessed mobbing behaviour in action. This fascinating and sometimes risky defensive tactic is common across many bird species and serves as an effective strategy for survival.
Mobbing is a coordinated defensive behaviour where birds aggressively harass a perceived threat, usually a predator, to drive it away. This behaviour can involve alarm calls, swooping flights, dive-bombing, pecking, physical attacks, and even defecating on the predator. Typically, smaller birds target larger threats, relying on numbers and persistence rather than sheer strength. Mobbing is most often observed near nesting areas, especially in species whose young are frequent prey. As well as defending against predators, birds that steal food may also engage in mobbing.
Some behaviours are similar to mobbing such as predator inspection, which can also reduce the likelihood of a predator attacking. However, unlike mobbing, predator inspection doesn’t involve harassing or physically interacting with the predator. Though predator inspection can escalate into mobbing, mobbing itself cannot be reversed into predator inspection.
Several theories have been put forward as to why birds mob, but most of the theories that try to explain mobbing behaviour can be divided into three main classes: altruistic behaviour, parental care and selfish behaviour. In 1978, the German ecologist and conservation biologist, Eberhard Curio proposed ten different hypotheses as to the purpose of mobbing. Some of the most studied hypothess are as follows:
One of the most widely accepted ideas about bird mobbing behaviour is the move-on hypothesis. This theory suggests that the main purpose of mobbing is not to harm or stop predators from hunting altogether, but rather to drive them away from a particular area. By persistently harassing a predator, birds make the location uncomfortable and encourage the predator to leave and search for prey elsewhere.
This idea aligns with observations that predators often abandon a spot after being mobbed. The constant harassment makes it hard for them to rest, hunt, or stay hidden. The move-on hypothesis is especially relevant for birds protecting their nesting sites, as it reduces the immediate threat to their young.
One study provided evidence for the move-on hypothesis by looking at how mobbing affected the powerful owl (Ninox strenua), a large nocturnal predator. Researchers studied which bird species mobbed the owl, how they did it, and how often the owl preyed on mobbing versus non-mobbing birds.
The owl was mobbed 35 times by seven out of 44 species of forest birds in a site made up of mostly open forest and a small area of rainforest. Most mobbing events involved one species, but occasionally, different species mobbed the owl together. The birds that mobbed regularly weighed between 4% and 26% of the owl’s body weight.
In about a fifth of mobbing events, the owl left its resting spot, and in more than half of the cases, it responded by calling out or watching the mobbing birds. The study found that mobbing was one reason the owl preferred to rest in the rainforest, where mobbing was less common compared to the open forest. The denser canopy in the rainforest made it harder for mobbing birds to spot the owl.
Of the 12 species of forest birds within the owl’s prey size range, six were regular mobbers and six were not. The owl hunted non-mobbing birds nearly 9 times more often than mobbing birds. While mobbing a large predator like the owl can be risky, it clearly helped the mobbing birds reduce the chances of being hunted.
Another hypothesis suggests that the mobbing behaviour and calls of prey species could attract another predator that preys on the predator being mobbed, effectively removing the threat.
To test the hypothesis, a team of researchers played the calls of light-vented bulbuls (Pycnonotus sinensis) near a taxidermy model of their main predator, the collared scops owl (Otus lettia). They used three different types of calls: the bulbuls’ usual call used as a control, a mobbing call directed at the collared scops owl, and a mobbing call aimed at a crested goshawk, a more powerful predator.
The results showed that when the mobbing call was directed at the collared scops owl, crested goshawks appeared and attacked the model. This did not happen with the typical call or the mobbing call aimed at the crested goshawk. The difference between the mobbing call to the collared scops owl and the other calls was significant.
These findings strongly support the idea that mobbing calls aimed at primary predators can attract stronger predators, which may then harm the original threat.
The cultural transmission hypothesis suggests that by observing mobbing behaviour, naïve individuals can learn valuable lessons about predators or dangerous locations. This helps them avoid these threats in the future.
For young birds, responding quickly and correctly to a predator’s attack is a matter of life and death, especially during their first encounter. In many social species, inexperienced juveniles learn how to react by watching older, more experienced individuals. However, the long-term benefits of this learning have often remained a mystery.
In a 2017 study, researchers uncovered how juvenile Siberian jays (Perisoreus infaustus) gain a significant survival advantage by watching adult jays mob their main predator, the goshawk (Accipiter gentilis). Siberian jays live in family groups of two to six, sometimes with unrelated nonbreeding individuals. Field observations showed that juvenile jays rarely come across predators on their own and, when they do, they don’t respond to predator models. But when they see other group members mobbing the predator, they spring into action.
Through a series of experiments, the researchers discovered that juvenile jays who observed adults mobbing a goshawk model were far more likely to survive their first winter. This increased survival gave them a better shot at securing a breeding position later in life.
Other theories about mobbing behaviour include
This suggests that mobbing serves as a warning to other animals nearby, helping them take action before they spot the predator themselves. By alerting others, the chance of survival for the group is increased.
This theory applies when an individual has already been caught by a predator. It says that mobbing the predator in this situation can distract it, giving the trapped individual a chance to escape.
According to this idea, when prey mobs a predator, it’s signalling that it has already detected the threat. The prey’s alertness, proximity to cover, or ability to evade an attack makes it less likely that the predator will succeed.
This suggests that mobbing can be a way for individuals to show off their fitness, demonstrating their ability to survive and avoid being eaten. One study supports this hypothesis, showing that male birds often increase their mobbing behaviour when conspecific females are present, especially when mobbing more dangerous predators. However, it remains unclear whether more frequent or bolder mobbers actually have more success in mating or are in better condition than others.
Mobbing is observed in a wide range of bird species, but some are particularly known for this behaviour:
Small songbirds – Tits, finches, blackbirds, and wrens frequently mob larger threats.
Corvids – Crows, magpies, and jays are highly intelligent and often engage in mobbing, sometimes even targeting humans.
Seabirds – Gulls and terns fiercely defend their colonies, launching aerial attacks on intruders.
Birds of prey – Falcons and hawks occasionally mob larger raptors such as eagles.
Birds most often mob predators that pose a direct threat to their nests or flock, including:
Birds of prey – Owls, hawks, falcons, and eagles are common targets.
Nest raiders – Crows, magpies, and squirrels, which steal eggs and chicks.
Mammalian predators – Foxes, cats, and even humans may be mobbed if they get too close to a nest.
