Last year I was mobbed eleven times. I was always minding my own business as I walked quietly along our woodland trails, but nesting birds didn’t see it that way. To them I was a predator, and they wanted me to move on.
Birds or other animals mob by harassing a common enemy such as a hawk, owl, snake, or even a human. They call loudly, surround the predator, and change locations frequently. Often they perform wing and tail movements called “distraction displays.” All of this commotion, usually by a pair of nesting birds, brings other birds in to see what is happening.
Douglas H. Shedd, who studied mobbing for ten years, recorded mobbing by 60 songbird species, five woodpecker species, two hawk species, and one hummingbird species. Gulls, terns, and other colony-nesting seabirds also mob.
Shedd wanted to find out why birds mob so he used a stuffed screech owl and recorded screech-owl calls to encourage mobbing from both summer and permanent bird residents. Summer residents, he discovered, stopped mobbing entirely at the end of the breeding season while permanent residents decreased but didn’t altogether stop then.
Black-capped chickadees were especially insistent, continuing throughout fall and winter even on a snowy January day at 25 degrees below zero! Shedd concluded that although birds may use mobbing to defend a nest or serve as a distraction display, their primary goal is to encourage predators to “move on.” Other researchers discovered that young birds can learn to recognize a predator by watching experienced birds mob.
Inciting birds to mob, researchers have recently learned, is an excellent way to find out what species of nesting birds are in the area. But instead of walking through the woods and depending on serendipity, as I have, they play recorded mobbing calls of certain birds, especially black-capped chickadees.
One study in Canada using black-capped chickadees identified 50 bird species and another, in Wisconsin, 24. During my walks last year from May 29 until July 15, 22 bird species mobbed and/or responded to mobbing calls by others. Many were repeat performers; others appeared only once. All except downy woodpeckers were songbirds and 17 were migrants.
A pair of northern cardinals started it. As I walked through a ten-year-old, recovering clearcut ravine, I heard a low warning call like a “rat-a-tat-tat” and then full-throated scolding calls from the cardinals. They moved toward me from out of the underbrush, and I knew they had a nest nearby. Instead of searching for it, I stood still. A pair of American redstarts, an eastern towhee, a rose-breasted grosbeak, red-eyed vireo, black-capped chickadee, worm-eating warbler, and a second male cardinal flew in for a closer look.
Best of all, though, was a totally unexpected bird, one I have rarely seen on our mountain–a yellow-throated vireo. Because of the date and habitat, it could have been either a very late migrant or a breeding bird since its preferred nesting territory–large trees in an open woods–existed nearby at the edge of the ravine.
The eight species I saw that day turned out to be the highest number I attracted at one time during my unplanned mobbing study.
On June 2, as I walked along Laurel Ridge Trail, I heard the cries of nestlings. About 40 feet off the trail I found a downy woodpecker cavity nest in a dead limb of a live chestnut oak tree. The parents erupted in full cry, bringing in a pair of hooded warblers, followed by a scolding red-eyed vireo, an ovenbird, and a worm-eating warbler. Afraid that I would attract a real predator to their nest, I quickly moved on.
Two days later a pair of ovenbirds scolded when I neared the rockpile turn-off for Lady Slipper Trail. In response to the ovenbirds’ distress, a blue jay screeched as he flew low overhead. An eastern towhee called. But the parents were the most persistent as they continued scolding and flying around me with their orange crests erect.
Ovenbirds, as ground nesters, are especially proficient at mobbing. On an overcast, foggy, Solstice day, ovenbirds mobbed me along Laurel Ridge Trail. A red-eyed vireo, four tufted titmice, eastern towhees, a ruby-throated hummingbird, a male scarlet tanager, and a black-and-white warbler came in to help. The titmice seemed as upset as the ovenbirds so they may have had fledglings nearby. I was particularly pleased to see the black-and-white warbler. Although I had heard their distinctive “weeza, weeza, weeza” often, it was the first good look I had had of one all season.
Ovenbirds were still mobbing on July 15, this time along the Short Circuit Trail. A female scarlet tanager and a red-eyed vireo flew in for a look, followed by a female common yellowthroat and then a male. Finally, after ten minutes of mobbing calls, a male towhee checked out the scene. His mate appeared a few minutes later.
Earlier, on June 28, the towhees initiated mobbing at the edge of the Norway spruce grove. A male towhee mobbed while a female sat nearby with food in her beak. A pair of chickadees, a common yellowthroat, and an indigo bunting added their scolding calls to those of the towhee.
Worm-eating warblers, another ground nester, frequently mob. On a splendid late June day, the sky scrubbed clean by a recent rain, I wandered along Black Gum Trail. Suddenly, a pair of worm-eating warblers, one with food in its beak, went into a frantic distraction-display, calling and fluttering close to me. In flew an ovenbird to look and scold, then a singing red-eyed vireo, and finally a pair of ovenbirds. I kept moving on, but the worm-eating warblers followed me, still scolding, for a couple hundred feet.
Field birds also mobbed me. On June 5, at the top of First Field, chipping sparrows scolded and flew toward me. I had one glimpse of a fledgling as both chickadees and a pair of towhees joined in with their cries. And, on July 2, as I walked the Butterfly Loop around First Field, I was mobbed by distraction-displaying and scolding indigo buntings, common yellowthroats, field and chipping sparrows.
Then there were the chickadees that researchers use to attract other birds. They mobbed me only once, on the Fourth of July. Six chickadees surrounded me and scolded, bring in a silent blue-gray gnatcatcher, a male downy woodpecker, and a red-eyed vireo.
Of all the birds that mobbed or were attracted by mobbing, the chickadees, at 11, topped my list, followed by nine towhees, eight ovenbirds, five red-eyed vireos, five common yellowthroats, and four each of worm-eating warblers, chipping sparrows, and tufted titmice. The other 14 species that responded three or less times were American redstarts, rose-breasted grosbeaks, cardinals, yellow-throated vireos, hooded warblers, blue jays, robins, ruby-throated hummingbirds, scarlet tanagers, black-and-white warblers, indigo buntings, field sparrows, blue-gray gnatcatchers, and downy woodpeckers.
Many of the species I recorded were the same as those found by researchers in Canada and Wisconsin. In one study, using the recorded “high-zee” and “chick-a-dee” calls of a chickadee mobbing a stuffed saw whet owl, researchers attracted 50 species in New Brunswick and 24 species in Quebec, and they proved that mobbing accurately measured the reproductive activity of black-throated blue warblers and ovenbirds.
The Wisconsin study using black-capped chickadee mobbing calls found that “all of the 24 species responding to the black-capped chickadee calls nest near chickadees and have frequent exposure to chickadees’ active predator mobbing” and concluded that “mixed-species mobbing may be an important factor in successful predator defense.”
Another study in southern Quebec, where researchers played the mobbing calls of both chickadees and red-breasted nuthatches to find out whether forest birds would cross openings to respond to the calls, attracted 37 bird species. Chickadees, red-breasted nuthatches, and red-eyed vireos responded the most to the calls but were reluctant to cross even narrow open areas. In fact, “When given the choice of traveling through woodland or across a gap, the majority of [birds] preferred woodland routes, even when they were three times longer than the shortcuts in the open.” This proved, researchers concluded, that “the presence of woodland links…may…significantly facilitate songbird movement.”
Encouraged by these uses of mobbing calls, the Cornell Lab of Ornithology decided to supply its citizen scientists participating in their Birds in Forested Landscapes breeding bird study with a recording of chickadee mobbing calls mixed with screech-owl calls. “Hundreds of participants across the country spent lots of time searching the woods, but they didn’t find many nests…If the timing [of playing mobbing calls] is right, some birds may approach while carrying food to their young. Others may come with their recently fledged young,” they wrote in their Spring 2001 BIRDSCOPE publication.