On a gray, chilly February day a flock of six black-capped chickadees is searching for the now-scarce insect eggs on trees at a forest edge. As they move in a loose group through the woods, they stay about 20 feet apart and give an occasional loud “chick-a –dee-dee” call, which probably helps keep flock members aware of each others’ locations. Then a new sound is produced, a high thin “zee-zee,” and a sudden change occurs in the behavior of the chickadees. All are frozen in place, feathers sleeked, and not even a flicker of the head detracts from their rigid immobility. Only the “zee-zee” sound continues.
What could have caused this remarkable change in normally noisy and conspicuous behavior? A northern shrike, a predaceous songbird with a strong hooked bill, has landed about 40 feet from the chickadees and is scanning the area with frequent head movements. The shrike is searching for its next meal, any unwary small bird or mouse. This time it goes hungry, for after about 10 minutes, it leaves. The chickadees continue their immobility for several minutes after the predator leaves. Suddenly one chickadee gives a loud “chick-a-dee” call, others join in the calling, and the chickadees resume their noisy wanderings in the woods. Danger has passed, at least for now.
One of the members of the flock may have been saved from becoming the shrike’s next meal by the chickadees’ early warning system. A sharp-eyed chickadee had spotted the shrike as it was flying into the area, and even before the predator landed, a chickadee gave the “zee-zee” calls (called high zees). These special calls are only given during times of acute danger when a predator is nearby, and they usually alert the whole flock to the presence of danger. The chickadees’ reactions of immobility are of great adaptive value, as the shrike probably has trouble detecting the dull-colored chickadees unless they give away their presence by movement. A motionless black-white-and gray chickadee can be very inconspicuous, even in a deciduous forest in winter. If the chickadees are in an open area near dense vegetation, they may dive for cover if the predator is detected while it is still some distance away. If, however, time does not permit this strategy, freezing in place is the best recourse.
High zees have some very special features that make this call very different from the other calls in the chickadee’s repertoire and adapt it for its function as a predator alert. The sound characteristics of high zees are very different from the “chick-a-dee” call, which is the normal flocking call and also the all-clear signal. High zees are one of the highest pitched chickadee calls (at about seven to nine kilohertz). Also, they cover only a very narrow frequency range and fade in and out without an abrupt beginning.
All these features make the call very difficult to localize. The predator can probably hear the call, but may not be able to find the caller, a feature that certainly works in the chickadee’s favor in the age-old contest between predator and prey. The call is also of very low amplitude, but this is no problem for the chickadees, as they are usually near enough to each other for the call to be effective. The predator is usually further away and low amplitude may contribute to the call’s difficulty of detection. The call, then, has design features which make it beautifully adapted for its function, alerting other chickadees, but making it difficult for the predator to use the chickadees’ warning system to find a victim. To human ears, high zees sound very faint and even ventriloqual. When I hear them, even when given by a chickadee near me, I have difficulty locating the caller.
Once danger passes, the chickadees resume activity, but they usually wait until several minutes have passed. A predator may return and it is best to be cautious. Then a chickadee, often an older dominant male in the flock, who presumably has acquired considerable experience with danger, gives the all-clear call, a resounding series of “chick-a-dee” calls, as a signal to resume activity. This call is in many ways the opposite of high zees–it has an abrupt onset and ending, is lower pitched than the alarm call, covers a wider frequency range and is louder. In contrast to high zees, this is a signal that can be heard over a considerable distance.
There are other possible explanations for alarm calling. While giving a call may seem altruistic, an individual taking some risk to save another, perhaps in reality it is a selfish act. Even if the flock consists of unrelated individuals, it may be in the caller’s best interests to have other flock members warned so that all escape detection. If the shrike or hawk takes an unwary chickadee, the predator may return to that spot soon. Since chickadees are rather sedentary, this puts all that live in the area at risk.
Understanding the chickadees’ warning system may help explain why chickadees flock during the fall and winter. There are advantages of flock life, compared to living alone during the non-reproductive season. For chickadees, many of these advantages are associated with avoiding being eaten. A flock has more pairs of eyes and ears than a single individual, so the chances of a predator being detected are much greater in a flock.
In my years of watching chickadees, I have always been impressed by their ability to detect predators while the danger is some distance away. While watching chickadees, I often hear high zees but seldom see the predator. Perhaps a sharp-shinned hawk has moved through the woods too quickly for me to detect. I have not seen it, but at least one chickadee has spotted it. Because the chickadees’ predator alert system works quite well, each individual chickadee may gain an advantage in that it does not have to devote so much time to be as watchful as if it were solitary. Group action such as mobbing may also be especially effective. For chickadees, there is truly safety in numbers.
Peter Dring is a retired nature biologist and phenologist who lives in the Land O’ Lakes area. To comment on this story, visit the “Outdoors” section of StarJournalNOW.com.
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