Swarming is how the honey bee species propagates. Hive population increases rapidly in the spring causing the bees to start preparations and eventually swarm in May or June. Swarming divides the hive when the queen takes off on a one way trip with half the bees to look for a home while the weakened colony is left to raise a new queen.
The bees preprogrammed instinct to swarm isn't exactly what the beekeeper likes to see just at the time hive populations are being built up in anticipation of the honey flow in June. Bees hanging on a tree don't produce honey for the beekeeper!

SPLITTING HIVES
Reducing hive populations is a way to "cool" the bees down this time of the year and thus postpone their urge to swarm. Hives are split by removing frames of bees and brood and placing them into another box which forms a nucleus to start a new hive. Bees require a minimum population of around 1000 in order to function as a colony and the nucleus (nuc for short) provides this and more. In practice, it's not quite as simple as it sounds because the bees and frames of brood must be removed from the hive without the queen. Queens are time consuming to find and when splitting hundreds or thousands of hives there isn't time to hunt up queens.

The queen excluder, designed to be placed between boxes, consists of a precisely spaced wire grid that the queen, because of her larger size, can't get through (see pictures). After pulling frames from the hive, all the bees are then shaken back into the hive and the frames are placed into another empty box as shown.

The queen excluder is placed on the hive and the box with the frames - less the bees - are then added to the stack. After replacing the lid, the setup is finished. In a few hours time, the bees will migrate up through the queen excluder to cluster on the frames above as shown in the last picture at right.

Worker bees spend the first several weeks of their life span inside the hive, never going outside, until they take their first flight and graduate to the field force. Once this transition has been made, they will spend the rest of their life flying whenever possible from sunup til sundown seven days a week gathering food, water and other essentials for the colony.
During the young bees time before flight, one of their principal duties is to attend to and feed the brood (larva). So it is these bees that migrate through the queen excluder to resume their duties with the brood now on the frames above.

It is the younger bees that make the most desirable split because they will most readily accept a new queen. For this and other reasons discussed, the method of splitting hives with the use of a queen excluder is preferred.

DRONES
Of the three types of bees, drones (male bees) have the best and worst life in the bee hive.
The queen and the worker bees are all female - the difference being that the queen is the only female with a complete reproductive system, having the ability to lay fertilized eggs. Virgin queens take a one time series of mating flights about 10 days after they hatch during which they mate with a number of drones while in flight. In this process they collect and store enough semen to fertilize all eggs laid for the life of the queen which can be up to three years. Fertilized eggs develop into worker bees; unfertilized eggs become drones. A worker bee has the ability to lay eggs but, not to fertilize them.
Drones have no stinger, gather no food, produce no direct benefit to the colony and are helpless without worker bees who feed them. The drone's only purpose is to mate with a virgin queen, after which they die. Drones, while having an essential role in the continuation of the species, are reared and tolerated by the workers only during times of abundant food supplies and are expelled from the hive to die in the fall and at other times of dearth.
Larger in size than the worker, drone brood is also larger than worker brood. Notice the drone brood (protruding) in the bottom of the picture on the right side of center as compared to that of the worker brood to the left.