What exactly is a Swarm of Bees?
Instead of swarm, people often use the word nest or hive when what they're really referring to is a swarm which is different to the nest or hive. The word refers to something quite specific, it is transient and quite different from an established colony. It's actually how bees procreate, it is probably more appropriate to think of it as the colony's 'child'.
In order for the species to prosper it isn't enough for the colony just to survive. It has to produce new colonies in the same way humans produce children.
Of course new individual bees are being created all the time, but the colony itself usually stays in one place. When the colony becomes overcrowded it's necessary to make more room and therefore the workers select an egg or several eggs to become new queens.
Just before the new queen is ready to emerge from their elongated cell, approximately half the work force gorge themselves on honey and old queen leaves the hive followed by the well fed workers. This is what we know as a swarm.
They alight on a tree or perhaps a wall. Often there can be more than 15,000 individuals, and is about the size of a football. Most of the bees stay clustered around the queen, some go out to scout for a new permanent home. There is an interesting article on the National Geographic site about swarm theory.
Sometimes a the cluster remains like this for a few hours, sometimes days. Very occasionally the bees decide to stay where they are and build combs exposed to the elements there in the tree.
Because the bees fill themselves with honey to sustain themselves until they find a new home, before they leave their parent colony, they are usually in a good mood. There's an old saying, "Swarming bees never sting.", IT ISN'T TRUE! Bees should always be treated with respect and removed by a beekeeper as soon as possible after they are noticed.
Since the world honey bee population is in crisis, I believe they should never be killed. In many places it is illegal to kill a colony of bees, unless a beekeeper has first tried to capture it, and then only by a licensed swarm pest control professional.
If they are not removed they may decide that their new home is in the roof of your house or a cavity in the wall. If bees are allowed to remain in such a location they will build comb and fill it with honey.
They maintain the temperature and humidity of their hive. If they subsequently die or are killed, the combs may melt in the summer heat and the honey will absorb water and begin to ferment. I'm sure you can imagine what happens when fermenting honey starts to drip down your wall or through your ceiling. It's not the best thing to enhance our decor.
I once saw a motel where two colonies of bees had been in residence in the roof for quite a while. An exterminator had killed both hives by spraying insecticide into the cavities. When I arrived the temperature was 100°F (38°C). There was honey dripping out of the end of the eaves into two five gallon buckets, it was a little late to call a beekeeper!
Sometimes a bees can be removed with a swarm trap consisting of a small beehive or other container in which is placed some honeycomb and perhaps some type of lure to attract them. Swarm traps are so much better than allowing bees to end up somewhere other than a hive or swarm box which can be removed to another location.