Watch along wind breaks, tree rows, especially in elevated areas such as  
hilltops. 
 
But, as many have said, unless you are near a really big breeding yard -  
such as some that I've seen in CA with thousands of nucs, with a few hives,  
you're probably not going to see/hear a drone aggregation area.
 
I mentioned radar, because it is usually not easy to find the  drones.  Nor 
is it necessary.  Best to be sure you've sufficient  drones - most breeders 
have a group of hives in the yard, each containing lots  of drones.  Don't 
rely on recruiting drones from feral colonies or other  beekeepers - supply 
you own.  The drones and queens know where to go.
 
Jerry

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