If you have more than a few drones, the congregation area is obvious by the buzzing and chasing. AS I recall, the best time to see the action is right after lunch in our area.
Ours just happened to be right over the nuc yard near our home and shop and the drones were chasing the swallows living under the eaves of a shed at midday.
The swallows blamed our cats and were dive bombing them, so the cats had to hide under the motorhome. It was quite a show.
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