Many authors have written about the new bird species discovered by members of the Lewis and Clark expedition (1804–1806), but the history of one species in particular – Oreortyx pictus (mountain quail) – has remained elusive. Primary sources confirm that Lewis returned from the expedition with a specimen (now missing), which he deposited in the Philadelphia Museum. However, for unexplained reasons, the specimen was overlooked by Alexander Wilson and other ornithologists. Here, I review primary sources from the expedition and its aftermath, including novel sources that resolve this long-standing puzzle.