Honeybees generally swarm once a year in the Spring, with the majority of the swarms in Colorado in May and June.
A swarm is a natural occurrence that allows the colony to multiply. The old queen bee leaves her colony and takes with her a quarter to a third of the entire colony. As they depart, the bees get really active and begin to fly in large circles in the air, creating what appears to be a "cloud" of bees. This phenomenon can be frightening to the unsuspecting passerby, and the noise of thousands of honeybees flying in circles in the air is quite impressive. They soon find a small tree or some other object to land on and begin to congregate. The honeybees cluster to form a large ball, which is usually elongated like a football (see photo). The queen is in the center where she is very well protected. The bees will stay in this swarm for 1-3 days while scout bees are dispatched to locate a new home. Once the scout bees find a new home, they go back to the swarm and do a bee dance. The bee dance tells the swarm distance and direction to the new home. Again, the bees start to get quite excited, they begin to fly in circles and the entire swarm moves to their new home. If you witness this, it is helpful to note the direction in which the bees fly. Some unsuspecting homeowner usually finds them within a day or so, at which time the swarm may have 2,000 to 10,000 bees. They can even withstand a rain shower, but if it gets too cold, they will die.
The best course of action is to call a beekeeper, like Schultz Honey & Wax, to catch the bees alive. It may seem easier to just let the bees go, but your neighborhood may not agree with this course of action. Do not exterminate the swarm of honeybees. Do not hit the bees with a stick or rock. Do not spray water on the bees. Leave the bees alone, but do take pictures. Several options are available to the beekeeper: hive the bees in a beehive, knock the bees into a box with vent holes or suck the bees into a special bee vacuum with a cage inside. This process does not hurt the bees. Once the bees are caught in a box or cage, the beekeeper installs the bees into a beehive, and within approximately one year, the hive will begin to yield honey.
There is usually a small fee charged by the beekeeper for time and gas. Some hobbyist beekeepers may catch a few swarms for free, but they can't travel far because of the expense. A small group of bees, the size of a baseball, may form after the beekeeper removes the swarm. This happens when the swarm is removed during the daylight hours and some bees get left behind. This group, however, is doomed and cannot survive without their queen. The weather may kill them eventually, but it may take a week for all of the activity to cease completely. The size of the ball should continue to get smaller until there are no bees remaining. The bees left behind are not dangerous, as they have nothing to protect.
Know this for sure: where there is a swarm, there is an established colony within 100 feet or so. The beekeeper generally inspects the area to locate the established colony where the bees came from. This includes looking at giant old trees which are hollow or homes which may have a hole leading to a large open space, such as under the roof, in the wall, in the floor joist, or around the chimney. A business card is generally left with the owner of the colony to have the bees removed.