On my walk to work the other day, I saw something unusual. There, about half a block ahead of me was a woman. Ok. A woman isn’t that unusual. The crazy part is that she was doing what I normally do. She was stopped on the sidewalk staring up into the trees. I could hear all sorts of grackles in the trees around there, making their squealing sounds. Before I got up to her, she began walking again, but I asked her if she had seen a hawk in the trees. “No,” she said, “Better than a hawk, it’s an owl!” She had an excited smile on her face as she walked off.
When I got to the tree where she had been standing, I looked up in time to see an owl take flight, followed quickly by about 10 other birds dive-bombing it- the grackles. Now it’s unlikely that the grackles could have done any real harm to the owl. What was it that the grackles were doing? The behavior itself is called mobbing. In a mobbing situation, other birds will seem to be attacking a larger predator. Why would you want to do that?
Well, there are a couple of potential benefits that come out of mobbing. One outcome is that the harassment makes it hard to be a successful predator. It’s awfully hard to concentrate on finding prey when a bunch of little birds keep flying into your face. Additionally, with everyone flying around and making a ton of noise, it makes it very challenging to be a sneaky predator. Your presence is announced to everyone around.
Northern Harriers attacked by mobs of other birds had a strong tendency to change their foraging paths. They would leave the area and go find somewhere new to hunt.
Now one of the cool parts of the story with the Harriers is that they were generally mobbed while flying. The owl was perched when the grackles mobbed it. Why the different mobbing patterns? Well, it certainly could relate to hunting strategies. Northern Harriers are active predators. They fly over large areas, gliding low to the ground searching for prey items. So a Harrier in flight is a hunting bird. The owl, on the other hand, hunts from a perch. They will wait in a tree to focus on a prey item and then swoop down to grab it. In both cases, then, the mobbing birds target the predator at a time when the predator is most likely to be hunting (and therefore most likely to get really frustrated!). That’s some real mob power.