They are not "singing"
They are letting all other comers know this is "their" space and to keep away.
years ago i did a study. (1982)
Record for a few hours, then play it back.
They would explore the source of the sound, then go away.
Took about a week to come back after no more recording playback.
Let me know how it works for you!
Okay. Thanks.
How did you record? With your phone?