YouTube is a good idea. Here's the video of Princess singing to me. Duck (a chicken named Duck that is) is in the nest box singing as well.
Awesome. It's so funny how similar it is to mine. Here: That same long "Bawk" is always followed by 5 of the short clucks (not quite as fast as Princess did). Your Princess makes the long drawn out bawk, then some medium, then many, many short clucks.
It started with my hens "Precious" & "Cuddles." I imagined they were saying
"TREATS treats treats treats treats treats" They would always follow me around making that same sound and soon many of my other hens joined in. For some reason, it was just me. The hens didn't make that specific vocalization when begging the kids for treats. For a few years, I figured it was because I did the majority of their care, so that's why they "sang" to me in that way. They know the daily morning routine and when I finish cleaning/sifting, I refill water, then put a bowl of food in the run. I figured they were just excited about the upcoming food bowl - after all, most of my hens are Orpingtons! Sometimes (if I'm busy and not acknowledging them) it can get pretty loud.
I got a book from the library about chicken communication, called
How to Speak Chicken. It was a fun read (for a chicken geek like me). You probably already know & understand most of the noises your flock makes. That morning song could be my name (in chicken language) and those original hens taught it to the rest of the flock.
So now I wonder if your hens do that same noise (long BAWK -followed by many short clucks) to you regularly. If yes, is that a generalized call that all chickens make for their #1 caregiver? Or, Could your lav orp have heard it as a chick when living here and is using a different version of it for you?
Edit to add: It could also be an "Orpington thing." My seramas make all sorts of noises - but not really that one. My EE & Leghorns live with the orps, and they will sometimes do that song for me. However, they are more likely to scratch about outside while I'm cleaning. It's the big, lazy orps who stand around me, chatting, while I'm cleaning the coop/run.