I'll have to try the auger hole trick.  FYI, for critical trials, we  often 
use bold, colored shapes attached to each hive - both the color and  shapes 
vary, so that we provide two types of visual cues.  
 
That said, I suspect that the drift to strong colonies may be   attractant 
to a pheromone - and maybe less aggressive guard bees.  Some  colonies let 
anyone in the door.  When we mark and release bees at a  distance from the 
beeyard (using a different color for  all 4 compass directions), we see the 
marked bees heading to the  strong colonies - and bees of all 4 colors end up 
in the strong colony).
 
Jerry

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