Clearly birds are master eavesdroppers, listening in on alarms sounded by other birds. So are other animals. In fact, one study showed that more than seventy species of vertebrates eavesdrop on alarm calls: Birds eavesdrop on other birds, mammals on other mammals, mammals on birds, and birds on mammals. In North America, chipmunks and red squirrels grasp the meaning of bird aerial alarm calls. In turn, chickadees understand the alarm squeaks of red squirrels and will take cover in response. Three species of lizards even attend to bird alarm calls. Yellow-casqued hornbills of West Africa can
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