Life isn’t easy when you taste a little like chicken. Recently, I watched a dozen crows harassing a bald eagle in Northeast Harbor. What had the eagle done to deserve such a comeuppance? He had almost snatched a duck. The eagle swooped into the harbor, trying to overtake a female mallard on the wing. The bird splashed into the water, just as the eagle was about to grab it. Thereupon, the duck “ducked” and the eagle barely missed. Before the eagle could cartwheel around for a second attempt, it found itself mobbed by a flock of angry American crows, which drove it off and saved the duck. The crows weren’t particularly interested in the duck’s well-being. They were merely expressing their own irritation. Crows are keenly aware that the eagle also considers them to be food and they do not appreciate it.
Mobbing is a behavior often seen, but little understood. It is plain that nesting birds will try to drive off an intruder. This seasonal performance persists until the youngsters are on their own. Two summers ago, I watched a merlin swoop through a sand pit hoping to pick off a bank swallow behind the post office in Medway. It missed, and the swallows angrily chased it to the horizon, probably all the way to Benedicta. Winter is also a time when birds do more mobbing. Years ago, I had occasion to be outdoors for several hours during one cold January day along the Bangor waterfront. Every 15 minutes a Cooper’s hawk would dash at the pigeons around the church steeples, usually missing, and then the crows would chase it out of town. In spring and fall when the birds are moving around over a greater area, perhaps migrating, mobbing is much less common. There’s no benefit to driving away a predator when you’re not planning to stick around anyway.
The first rule of mobbing is: don’t get eaten. You’ll never see a duck or a dove mobbing a predator. They’re too fat, too slow and too tasty. Mobbing birds don’t have to be faster than the predator, but they do have to be more maneuverable. That’s why you are more likely to watch small birds mob a lumbering red-tailed hawk than a quick peregrine falcon. The second rule is: bring friends. Multiple birds can keep a nimble predator off balance, preventing it from using its advantage in speed, size and agility. In fact, that’s probably one reason why mobbing works. A successful raptor relies as much on stealth as speed. It’s hard to get the drop on your victim when you’ve got a dozen irate crows on your tail.
Oftentimes the elimination of surprise is enough. Smaller birds such as chickadees, nuthatches, titmice and kinglets will gather around an owl and scold. Warblers and thrushes will join in, but with a little less enthusiasm. They will keep up the din until the owl realizes the chances for surprise and lunch are both gone. Researchers suspect that the scolding and mobbing behavior also serves to educate the youngsters that this is a foe deserving of some distance. An owl in daylight poses little threat to songbirds, so the expenditure of energy to drive it off makes scant sense unless there is an educational motive.
The entire corvid family — which includes jays, crows and ravens — are mobbers. They even mob each other, since they know their own relatives to be notorious nest robbers. I see crows mobbing ravens often, but only in the spring. Birds that breed in colonies are more likely to mob, since they have plenty of help. You can experience this for yourself. Just walk into a colony of nesting terns and be prepared to run for your life.
Experienced birders use the behavior to their advantage by mimicking sounds that are associated with mobbing. This draws in songbirds that might consider joining the fray. Even birds that aren’t likely to harass a predator will come to see what the fuss is all about. Owl imitations can do the trick, and some folks can do a near perfect simulation of a barred owl’s hoot. Small owls are more likely to prey on small birds, so saw-whet and screech owl imitations can be effective. In the west, birders are apt to imitate pygmy owls.
Clearly, mobbing works. The eagle got only one nip at the duck and left with a fowl taste in his mouth.
Bob Duchesne serves in the Maine Legislature, is president of the Penobscot Valley Chapter of Maine Audubon, created the Maine Birding Trail and is the author of the trail guidebook of the same name. He can be reached at firstname.lastname@example.org.