[techmant]

I was developing a theory about the same thing. My thinking was that drones might put out a pheromone of their own and when it reached a certain concentration level that it might be a part of the swarm triggers. Well I was proved incorrect by my observation hive that had very few if any drones. They swarmed just the day before I was going to split the hive.They had been building swarm cells so I knew they were going to swarm...just didnt have my date exactly right. There were no drone brood either. There may have been a few drones from my other hives freeloading but nowhere near enough to matter.
This observation hive was started last spring and had overwintered with the same queen. I know this one incident not definitive but it leads me to believe that drones have little to do with swarms.