I'll go with "congregation" since that is the term that has been used, and everyone understands it.

I checked the etymology and it clearly supports the usage IMO, modern and local distortions of language notwithstanding

from L. congregatus "flocking together," pp. of congregare "to herd together, collect in a flock, swarm; ...

and 

c.1400, from L. congregatus , pp. of congregare  "to herd together," from com-  "together" + gregare  "to collect into a flock, gather," from grex  (gen. gregis ) "a flock." 

>Location along tree lines but also in open where thermals may appear.

I wonder if it is not just that simple: drones idle around in an area where the air rises up and makes staying aloft easier.

---
I recall seeing what appeared to be tiny drones congregating above me one time while I was resting on a bench.  The bees were so tiny I could barely see them -- assuming they were bees

I wrote this on July 2nd, 2000

As I looked up, I discovered that there were insects that looked like small bees circling under a tree above me.  They were endlessly making random circles of about four feet diameter within an eight foot area 10 or twelve feet off the ground.  Every so often something would fly through the space and the the group would converge on it and follow.  The action was amazingly fast -- too fast for my eye to follow.  I never did see a comet, or any insects falling, but I supposed that I was seeing a drone congregation area for leafcutters or one of the mostly unnoticed thousands of bee species that abound at this time of year.

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