Birding by ear is a skill that can take years to master, but is worth the wait—it compliments the visual identification process in many ways. An advanced birder can walk through a forest or field and record bird species without actually seeing them. In most cases, a bird's song is diagnostic—you can identify the species from its song or call. For some groups made up of similar-looking birds, such as the Empidonax flycatchers, their song is the surest, or possibly only, way to identify which species you've encountered.
Rails, bitterns, and other elusive species may not offer you a glimpse of them, but they'll often vocalize behind dense thicket or cattail edges. Beyond the simple enjoyment of these calls, vocalizations from hidden birds can be helpful when conducting surveys. Birding by ear is also important during events such as a Birding Big Day, when one tries to identify as many bird species as possible in a single day.
A good tip to remember is that most groups of birdsong can be categorized into themes; most flycatchers sound alike, most vireos sound similar, and so on. Learning how a bird's song is sparrow-like or wren-like is one of the best ways to narrow down the possible species and identify a concealed bird. Though there will always be exceptions, when a song is diagnostic to a particular species, you can learn and recognize it. The waterthrush upstream is a Northern or Louisiana, but if it sings and you know the song, then you'll know which one it is.
Virginia Rail photo contributed by Mike McDowell, an avid digiscoper, amateur naturalist, and Eagle Optics employee.