Went to take off 4 supers from my strongest hive. The clearer board (rhombus, split into two with half at each of two corners) had been on for 24h...my usual method, works a dream normally. The air above the hive was full of bees, the sound was incredible, and there were hundreds of bees trying to get in under the roof. As I took the supers off (full of bees) a swarm cluster started to form up on on of the fence posts at the apiary. When I got to the clearer board, I found the holes leading to the rhombus (around 50mm diameter) completely clogged with dead bees.
It took about an hour to clear all the supers manually and put the hive back together again. As I left, the swarm cluster was still there but the hive was back to normal and the air was no longer thick with bees. All of my colonies had been checked with no signs of swarming or swarm preps. Been trying to work out what was going on...yet another mystery (to me at least!). I might have a gander later today to see if the cluster is still there.