Normally honeybees don't start swarming until May at the earliest; it takes a few months for the colony to regain their numbers from the winter losses and to get large enough to trigger the swarming instinct. This one was very early and came from the hive of a friend of ours who had left it alone since last summer due to illness. The result of this appears to be a very large and vigorous colony which has swarmed early; quite an advert for natural beekeeping!
Swarming is honeybees natural process of reproduction. The queen lays so much brood that the colony decide there are enough of them to divide into two. The workers then develop some eggs into future queens and while those special larvae are developing the old queen takes off with up to half of the hive in tow, to find a new home. That is the swarms that are seen.
Many swarms land in trees - like this one - and then die of the cold; very few find a natural suitable home such as a hole in a tree-trunk. Therefore, a beekeeper is doing a very useful service by capturing a swarm and giving a good man-made home.
This was a tricky swarm to deal with as it was wrapped around a sturdy trunk that I could not cut. I had to gently scoop out the mass of bees, with my hand, into my nuc' box and then leave the box overnight in the hope the rest would decide it made a better home. Luckily they did and had all moved into the box by the next day and are now safely housed in a hive at rosybee.