When Swarms of Birds Attack Other Birds, They are Using a Mobbing Technique

When a flock of crows began antagonizing an owl in my neighborhood, we could hear the commotion from blocks away. It was late morning, a time when owls typically sleep.

Dozens of crows swarmed the owl as it moved from a tree branch to a rooftop to a fence post. The owl seemed old and was missing an eye, and at times it seemed as though it was bracing itself when the crows darted close.

The crows antagonizing the owl were exhibiting mobbing behavior. In the past 50 years, more scientists have become interested in researching mobbing behavior, and they say it can be an important defense against predators.

What Is Mobbing Behavior in Birds?

Mobbing typically occurs when prey realize a predator is dangerously close. They work together to harass the predator and drive it from their territory. Mobbing can occur among other species including mammals.

Mobbing typically involves both vocal and physical posturing. Crows, for example, make loud cawing sounds. These sounds help alert other crows to join in, and the noise can also add to the target’s sense of confusion or alarm. Physical posturing can include chasing, dive bombing or pecking.

Which Birds Are Known for Mobbing?

(Credit: karen burgess/Shutterstock)

Many birds that mob can be found in backyards across the country. Blue jays, chickadees, crows, and mockingbirds are all examples of birds that will engage in a predator pile-on.

“I think it happens more often than we are aware of; we have to learn to recognize sounds,” says Holley Short, the shorebird program manager for Audubon Florida in St. Petersburg.

Mobbing will sound like cawing or angry chirps, distinct from typical birdsong.

“I have blue jays in my yard, I can hear them if they perceive a predator,” Short says. “They are trying to get that hawk out of their territory.”

Bird gangs typically mob predators such as hawks or owls. While some birds will mob any threat to their nests or territories — including humans who get too close — birds like crows and mockingbirds have recognition abilities and will target known threats, Short says.

When a flock of birds mob a predator, it is possible for different species to work together to harass a target.

“Some of these species like crows or jays, if they get started and you go and watch and listen, you can see other species join in,” Short says. “In these instances, I’ve seen cardinals join in, red-bellied woodpeckers and tufted tit-mice.”

Mobbing can also draw the attention of other species that also fall prey to predators.

“I’ve even seen a squirrel come out in response to all these calls and mobbing behavior to see what was going on,” Short says. “It sat on a branch and watched the hawk as it was mobbed by birds.”

Benefits and Risks of Mobbing Behavior

The benefit of mobbing is that dozens of birds can work together to drive a predator from their territory. This can help protect the young, but it can also leave nests vulnerable to attack.

While the flock is busy mobbing a predator, nests can be left unattended. In Florida, Short says that least terns leave their chicks and eggs vulnerable to opportunistic predators when they mob crows.

“It can be risky because even though the birds are mobbing, they are not really the ones that are at risk from the crows,” Short says. “The crows won’t attack the adults, but when [the least terns] are mobbing, they are leaving the colony unattended.”

Mobbing could also be risky if the much larger predator snaps back at its adversaries. Mobbing is a behavior that demands significant energy, and scientists have observed a seasonal effect in which birds are less likely to mob during the winter when resources are scarcer.

Can Birds Mob Humans?

(Credit: Jen Oswald/Shutterstock)

Birds will mob any threat to their territory, especially when they deem their young are at risk. The black skimmer, for example, is a black and white coastal bird with notable orange legs and orange-and-tan bills. These beautiful birds, however, will mob humans who are strolling on the beach and come too close to their nest.

Wildlife authorities often put up signs and ropes to direct people away from the nests. Short cautions that people take such warnings seriously.

“We like to tell people to watch out for rope and nests. We want to make sure you don’t step on anything, but you also don’t want to get mobbed,” she says.

How Long Does Mobbing Last?

Mobbing lasts as long as necessary. “It will last until the threat moves on and out of the territory,” Short says.

For example, humans who stumble upon a black skimmer’s nest will hustle out of the area. Predators, however, might not evacuate the area when mobbing begins. They might need to rest, or they may try again to raid a nest or attack an adult.

Mobbing provides a short-term solution to eradicating a potential threat. At this point, scientists can’t be sure if the predators learn that certain areas or flocks are off-limits after a traumatic mobbing.

And of course, researchers also don’t know whether other wildlife — like the squirrel Short saw watching the mobbing — are pleased when they see one of their predators driven from their territory.

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Advances in the Study of Behavior. Chapter One – Mobbing in animals: A thorough review and proposed future directions
Shorebird program manager for Audubon Florida in St. Petersburg. Holley Short
Frontiers in Ecology and Evolution. Mobbing behavior of songbirds in response to calls of an ambush-predator, the Northern Pygmy-Owl

Emilie Lucchesi has written for some of the country’s largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor’s degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, “A Light in the Dark: Surviving More Than Ted Bundy,” releases October 3, 2023 from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.