Quote from "Five Hundred Points of Good Husbandry", by Thomas Tusser:
"Take heede to thy bees, that are readie to swarme,
the losse thereof now is a crownes worth of harme:
Let skilfull be readie and diligence seene,
least being too careles, though losest they beene."
Book Footnote: "The Proverb says, 'A swarm in May is worth a Load of Hay.' - T.R."
In May, the amount of nectar, honey, and bees grows almost exponentially. When the colony feels the hive is becoming too full, the worker bees will create a set of new queens. The new queens will emerge and fight to the death to see you will remain as there can be only one. The colony will then take the old queen and about one-quarter of the worker bees and leave the hive to find a new residence. This is called swarming.
Swarming was important in the middle ages as this was the easiest way to start a new hive. Losing a swarm was very costly to the beekeeper. It meant one less hive for that season. The remaining colony takes a while to rebuild its population. This slows down nectar collection and honey production. The swarm would eventually increase its numbers, collect nectar, produce and store honey, and possibly swarm again.