A short recording, with uniform background noise, a half second or so of plain background noise at the beginning of the recording, and the target sound standing out clearly from any background noise (not overlapping with other birds or distinct sounds). Steady background noise is OK even if it’s fairly loud, as that can be filtered out, but intermittent noises like other bird calls, the swish of a sleeve, footsteps, dripping water, barking dogs, human voices, etc can confuse the app.
