Mysterious "Mobbing" Behavior of Crows and Owls

Nature Notes

By Patricia Martin

This owl is at home in his tree top perch. (Photo courtesy of Greg Main)

Spring is finally here. The boats just began running a few days ago and workers are starting to return. Crocuses have been popping up in my yard. Golden Eagles and Sharp-shin Hawks have been spied on their northerly migration and Sand-hill Cranes have been seen and heard. This is always an exciting time of year to watch for the changes.

This owl is at home in his tree top perch. (Photo courtesy of Greg Main)

This isn’t to say that winter didn’t hold its share of surprises. In my last column, I wrote about one northern bird, the Great Gray Owl, that decided to spend its winter in the more hospitable environment of Mackinac. Another owl appeared this winter, from the arctic and sub-arctic, the smaller Boreal Owl. In Europe, where they’re also found, they’re known as Tengmalm’s Owl. As a friend of mine describes, it kind of looks like a half-sized Great Gray. Even in their native environment, these birds are seldom seen, as they’re solitary, usually nocturnal, and conceal themselves close to the trunk of a thick conifer during the day. Living as they normally do in the far north, they’re unused to humans and seem to have little fear of people.

The Boreal Owl is only eight to 12 inches in length with a wingspan of about two feet. It has a whitish gray facial disk with a black border around it. Like the Great Gray Owl, the eyes of the Boreal are yellow and it has a pale bill. Its head has a flattish top with feathers that are brown or grayish brown with white spots. Most of the feathers on its back are dark brown with heavy white spotting. Its chest is whitish with chocolate brown streaking. The call of the Boreal Owl resembles high-pitched ringing ting, ting, ting, ting and it also emits a brief series of hoo notes.

Like the Great Gray, this owl occasionally comes south during the winter, probably owing to a decreased food supply (rodents, etc.) in the north. It is interesting that it is usually females that are found far south of the normal range during the irruptive winter and they’re known to make use of barns and other buildings for shelter (irruptions are southward autumn invasions by northern birds caused by lack of food). Their main diet consists of rodents and other small mammals and rarely small birds. These birds often hide food in caches like tree forks and woodpecker holes (they also nest in them). If the stored food is frozen, the owl will assume incubating posture to thaw the prey.

Another bird that was commented on frequently this winter was a crow. Now, crows are very common on the Island these days, though 100 or more years ago, Ravens were more often seen. This crow is a bit unusual, however. It has white feathers on its wings. I’ve been asked about it by several people in town and after doing a bit of research, I’ve concluded that this is just a Common or American Crow, which perhaps had a genetic change to the cells that produce feathers on its wings, causing white feathers to grow. These feathers stand out starkly against the rest of its feathers. As most of you know, the American Crow is a black bird from bill to tail with a purple blue sheen to their feather. In the long run, the white feathers may be a determent to the bird, as it makes him stand out and be more visible to predatory birds. Crows are extremely intelligent birds, which are omnivorous, eating everything from insects, small reptiles and mammals, eggs, grain, fruit, garbage, and carrion. I have a friend who gets rid of all of his leftover food by throwing it out to Hector and Heathcliff, the two crows who hang around his house. Crows are even getting into the feel of the spring season. A couple of days ago, I saw a crow collecting dried grass to put into its nest.

Now, you may wonder why I’m writing about crows and owls in the same column. In the last week, two people have mentioned seeing an interesting interaction between the Great Gray Owl and American Crows. The first occurred at Croghan Water, where the Great Gray was being dived on by a flock of crows. The second occurred on the West Bluff, where the Great Gray Owl was sitting, eating a dead crow. A huge flock of crows descended on him, making a racket with their loud cawing cries. The witness who saw it said that it reminded her of an old movie in which hundreds of birds all descended at once (perhaps Alfred Hitchcock’s "The Birds"). This seems like a rather odd behavior, as the owl is at least twice as large as the crows and is armed with a deadly sharp beak and talons. This action is logically enough known as "mobbing" and a number of species of birds engage in it, including chickadees, warblers, and gnatcatchers. It is not uncommon to see a group of blackbirds or swallows chasing a hawk or eagle. In fact, a birdwatcher friend of mine says that whenever he sees a group of blackbirds or crows diving at a tree during the day, he often looks to find an owl perched on a branch. The behavior is most commonly believed to be an anti-predator strategy, though it’s not fully understood.

Mobbing tends to occur most intensely on breeding grounds. It may function to divert the predator from the area where there are fledglings (become decoys if you would) or perhaps to confuse and annoy the owl, hawk or whatever predator, in hopes of getting it to move along. There is experimental data that demonstrates that birds learn from each other which predators to mob. It may be that one function of mobbing is educational, to teach young birds how to identify the enemy. The noise and movement of mobbing may also function to alert other birds to the presence of the predator, either to get them to join in the mob or to protect them, giving them the opportunity to get away. The original members of the mob may gain the direct benefit of driving the predator away or may gain an indirect benefit of protecting birds that are related to it.

One of the reasons mobbing seems to work is because of the type of hunting that raptors (hawks, eagles, and owls) do. These birds often do their hunting by sensing, by sound or sight, their prey, be it bird or mammal, and swiftly, and often silently diving down upon the victim. Surprise is most likely an essential element in raptor hunting. Without the element of surprise, these hunting birds have a lot harder time getting breakfast or supper and it’s awfully hard to sneak up on anything when a bunch of crows or other birds are screaming at you.

Last fall I saw this tactic work very well against a Red-tailed Hawk. This hawk was perched on a branch on the bluff near the Fort. It began with two crows diving and cawing at it. It flew up a little bit and tried to settle in another tree. By this time more crows had joined the fray. The hawk flew again. This harassment of the hawk continued for 10 to 15 minutes, until the Red-tailed Hawk simply gave up and flew away.

This spring, look for this mobbing behavior in birds and keep an eye out for the spring and summer visitors who should soon be flocking to our shores.

Trish Martin is a year-around resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.