Urban Beekeeping: The story begins...
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Bees propagate their species by the splitting of the colony.  This tends to happen once or twice a year typically in the spring depending on their rate of population growth and the amount of space capacity in the existing hive to grow.  The colony will split after the queen decides to go, leaving a new queen or set of queen cells behind in the existing hive.  It is always the older queen that leaves, and about half the colony will leave with it.  When this happens you can observe this as a giant flying swarm that will gather outside the hive, and then move as a cloud in masse to some new location typically nearby.  Often they will clump around the queen at some intermediate location until scouts can find a suitable home that provides adequate shelter to establish the new colony. This is when these bees are most vulnerable as not all the time are they able to find a good location for a new home. A hollowed-out tree is one good type of spot, while something more exposed may not be adequate to preserve colony humidity and temperature requirements of the brooding area (cells that hold bees in their pupae stage) through the extreme weather conditions.  It should be noted that when honey bees are swarming, they are not dangerous. This is because they have not yet established a new home, and thus are not territorial.  So it is possible to stand in the middle of a swarm and observe this fascinating act of nature, somewhat like standing in the eye of a tornado.  The biggest hazard is likely to be for some bees to get caught up in your hair, and getting stung as you fumble to try to get it out. Thus the best policy is just to remain calm and allow the bee to work itself out on its own.

Beekeepers tend to also get new colonies by learning about such swarms and then collect the swarm into a new hive box or container for transportation to a new location.  Sometimes beekeepers will advertise their phone number on some bulletin board or through some local gardening clubs in case someone has encountered a swarm that has decided to land at some inconvenient location.  I have seen and photographed Mojgan capture a swarm with a fellow neighbor and recent beekeeper convert (Chuck) on a couple of occasions.  The trick is being able to get close enough to where the swarm has temporarily clumped with the queen at the center, typically on some tree branch, and then placing some large bucket underneath it in which to transfer to a more permanent hive.  By giving the branch a swift jerk, the main clump of bees will lose its grip from the branch and fall into the bucket.  You then put a lid on the bucket or container, but with a hole a couple of inches in diameter in the lid for the additional swarming bees to enter, as well as to provide temporary ventilation.  They will not leave assuming the queen also fell into the container. Ideally you should leave the bucket at this location until dusk for the rest of the bees to enter and gather around the queen (they tend to hunker down in a hive for the night).  After that you can cover the hole, tape the lid, and transport the new swarm to a permanent hive at a new location.