This is going to be somewhat detailed so that I can hopefully get well informed responses. On Saturday, 4/1 a friend and I caught a swarm of bees from his fathers hive in Richmond,Va. We added three brood frames (9 1/8") worth of bees and one intact brood frame (brood, honey, pollen, bees and one intact queen cell) in a brood box with the swarm. Sunday,4/2, I put a med. super on with (9) drawn comb frames and an entrance feeder. Monday afternoon,4/3, my son finds a swarm of bees on our split rail fence in the backyard. This swarm is equivalent in size to approximately 3/4 of the bees we brought back from Richmond. My other hive has no swarm cells. I checked the Richmond hive and the swarm cell that was in there is still intact. So, I caught the swarm and dumped them back in with the Richmond bees ( I felt that was where they swarmed from). Within 2 minutes the swarm is fully formed back on the split rail fence. I put them in a nuc box and added an entrance feeder and they are now content. What I can not figure out is, why would they swarm when they had honey, brood, pollen and plenty of room. Could it have been a seperate swarm from who knows where, since they vacated the hive so fast?