Listen for singing whistles. The singing whistles of a unicorn are difficult to distinguish from birdsong. Experienced birdwatchers will have an easier time detecting these high-pitched sounds made when a unicorn forces air through its teeth or partly closed lips. The effect is not unlike that of the powerful whistling sound used by Manhattan doormen to summon taxicabs. In Inheritance of a Sword and a Path (2005), Douglas Van Dyke Jr. refers to unicorn language as the "whistle-song." He describes herd leaders making distinctive calls "like a variety of whistles set in a musical tone." He goes on to explain how the leaders use these whistles to guide a herd in motion: The noise was very different than anything that a horse might utter. Unicorns did give voice to a distinctive variety of singing whistles to communicate with each other. The herd leaders, which were always matriarchs, used the whistle-song to guide the rest of their followers. . . . The echoes of the song trilled and reverberated as it guided the direction of a hundred sets of unicorn hooves. . . . [T]he herd . . . switched directions on the run, flowing one way then striking a new direction