I keep honey bees in the garden. April to June is the period known as swarm season. A colony swarming is the way in which the bees increase their colony numbers. The queen and all the older flying bees in the colony flee the old hive, leaving behind younger nurse bees and a number of developing queens who will emerge in a few days time. One of these will head up the existing colony (usually the first to emerge, as she'll kill all the others before they emerge). The swarming colony flies a short distance initially and will typically settle in a cluster on a tree branch or similar. Scout bees will then look for a suitable permanent home. The swarms typically move again within a couple of days, at the most. The best time to catch them is before they move again. Despite looking very scary, the bees who swarm have been engorging themselves with honey prior to doing so, and are very docile as a result. If anyone spots a swarm, phone the local beekeeping association and they'll have a team of local volunteers on the swarm list who will come and retrieve them.