 Deer are highly unpredictable, skittish animals whose normal reaction to an unfamiliar sound is to stop, look and listen to determine if they are being threatened.If the whistle gives them advance warning, they may freeze on the roadside, rather than running across the road into the path of the vehicle.In Australia, a different product, with electrically powered speakers (Shu Roo), is used to decrease collisions with kangaroos.