Blueblood, you break my heart. What a nice swarm that could have been. Like the proverbial fish that got away. But at least you got a video.

I have never ceased to be amazed at how difficult it can be to spot a swarm clustering on a tree. I can have them right in front of my eyes and not see what I am seeing. But to see them fly away once you've almost got them, that is really rotten luck. :'(
When I go for a swarm, it is always with a hive, and most important, one or more built frames. I think they find them irresistable.