Tropical rainforests have evolved over millions of years. Birds can't see insects moving under the thick layer of leaves that covers the ground. But when army ants go out foraging in their hundreds of thousands, they make a humming noise that flushes other insects from their cover. This, in turn, gives the birds an opportunity to eat.
原來熱帶雨林茂密，經幾百萬年演化，在肥厚落葉層遮蓋下，鳥類根本找不到昆蟲蹤跡，只有當軍蟻出外覓食，數十萬隻「大軍」所到之處，發出嗡嗡聲響，蟲兒們驚嚇跳躍之際，鳥兒才能伺機捕食。