This is interesting. I’ve been dabbling in bird song with Merlin recently. I didn’t know you could use Audacity to see audio as a Spectrogram. This post shows how.

I’ve only just head of nocmig.

The nocturnal equivalent of visible migration watching, and typically employs sound recording equipment to capture the flight calls of migrating birds. 

nocmig – Recording nocturnal bird migration

Which looks/sounds interesting. Could be a bit of a rabbit hole: Guest blog: A DIY low-budget NocMig parabolic microphone, Although I don’t think my diy skills are up to this.