I watched a video on traditional bee keeping (http://www.youtube.com/watch?v=g7M1X...feature=colike) and the German catch their swarms right at the entrance to the hive. The narrator said that the bee keepers recognized the swarm as it was preparing to leave the hive. They slap a net over the hive and catch the swarm as it exits the hive.
What do bees preparing to swarm act/look like? How do you recognize the pre-swarm behavior?