As we all know, the “best” ornithologists and birders are those with both the ability to hear bird sounds well AND the ability to remember them and distinguish among various species. Others of us are in awe of people with these phenomenal gifts, and can only wonder why we can’t do the same. Most birders simply rely on tour guides to find and identify birds vocally, and while there’s nothing wrong with that, there ARE ways you can be more self-sufficient with aural ID.
High-register hearing loss (presbycusis): Typically, loss of ability to hear high pitches is an age-related, non-correctable condition. It is also often related to damage suffered due to long-term exposure to excessively loud noise. Moderate presbycusis will result in the loss of ability to locate some warblers, kinglets, creepers, flowerpeckers, etc. but not most larger species. A severe case is debilitating for a birder, to whom it seems that the forest is nearly silent, and almost everything has to be sought visually. In rare cases, the condition may be lifelong; Rasmussen had moderate high-register loss in youth, and the loss is progressively becoming more severe.
What can you do? Most hearing-aids are made only for help with sounds in the range of human speech. A few models use the same principle as the SongFinder (see below) but even these do not help above 8 kHz. The "SongFinder" (http://www.nselec.com/) may be the answer. The SongFinder lowers high pitches without slowing them down, so you can detect the bird and hear its pattern and rhythm, although the exact sound of the bird is changed somewhat to sound “harder” by being lowered. The device is housed in a small plastic box that one may attach to a belt, and connects to small headphones with microphones that also receive the resampled, lower-pitched sound. Lower-pitched sounds are not affected, and as the device is not primarily amplifying sounds it does not cause discomfort when, for example, a Carolina Wren sings nearby. Rasmussen’s experience is that the SongFinder makes a major difference in the number and variety of species that she can locate. The downside is that it’s another item to deal with, and the headphones mean you can’t monitor input when taping with headphones (unless you carry both and switch back and forth). But, for severe presbycusis, it’s well worth it!