If you've read Moby Dick, or seen the movie, you'll know what a crow's nest is . . . but did you know it originally had crows in it? According to the US Navy's Origin of Navy Terminology:
The crow's nest was situated high in the main mast where the look-out stood his watch. Often, he shared this lofty perch with a crow or two since the crows' cages were kept there: hence the "crow's nest."