Swarming is the act of colony reproduction, and a swarm leaving a parent colony is looking for a suitable “cavity” to establish a new nest.
You are more likely to discover a swarm between May and July.
When a new swarm emerges from the parent hive, the air is full of thousands of excited, flying bees (Fig.1) searching for a place to cluster (Fig.2).
After flying a short distance, the swarm clusters together temporarily in a place where “Scout Bees” can look for a new place to nest.
Clusters can remain for a few hours or a few days, but the longer it sits on a tree in your back garden, the more likely it is that the bees will take up residence nearby, if not in your house, shed (Fig.3), garage, old tree or other obscure places!
Beekeepers are generally called to swarms that are clustering and this is really the only time it is possible to collect them easily. Once they have taken up residence in their new location they are very difficult to remove.