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An ultrasonic device works on wild animals much the same way that a dog whistle works on dogs. It produces a sound that hurts their ears, or just plain frightens them depending on the species, and thus they stay away from it. As for why they don't avoid the sound of the car engine, there are no natural predators that sound like cars, while there are natural phenomena, such as flowing water, do sound similar to cars, so animals have no instinctive fear for the sound of a car. Make your car sound like a roaring mountain lion and then they'll stay away from it.
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