What is the most reliable identification method when visual cues are difficult to observe?

Answer

Associating the distinctly musical, fast, rising and falling song sequence

While visual marks like primary projection exist, learning to associate the bird's unique, musical song sequence with the Willow Warbler is cited as the most dependable skill for identification.

What is the most reliable identification method when visual cues are difficult to observe?
speciesbirdanimalwarbler