They like to nest in bird boxes
During the summer beekeepers get many calls from non-beekeepers about bees in bird boxes. In the vast majority of cases these are bumblebees, not honey bees. Bumblebee queens look for a suitable nest when they emerge from hibernation in the spring, with many species favouring old mouse or bird nests, hence the problem with bird boxes.
The Tree Bumblebee Bombus hypnorum has become established in this country since it was first noticed in 2001 and prefers nest sites above ground. There has been an increase in the number of bumblebees making nests in bird boxes in recent years, many being Tree Bumblebees. They are quite aggressive in defending their home and will sting more readily than other bumblebees, even without provocation.
Bird boxes are ideal for them as they are the right size, easily defendable and often have an old birds nest waiting for them to use. Queens often establish their nests in the spring before birds do, meaning that birds can't use the boxes for the purpose they were put up for. You won't have the benefit of observing the flurry of activity that is seen when the young are being fed. For that reason I suggest that all bird boxes are cleaned out each year during the winter and if possible by the end of February.
All species of bumblebees will vacate their nests towards the end of the year, although timing depends on the weather and the species, but in general they are vacated by the end of October. Queens and drones (males) are reared towards the end of the season, with the drones dying and the queens feeding to build up their fat bodies, ready for hibernation. In the spring they become active, look for a nest site and the cycle is repeated.
I suggest it is a good idea for local BKAs to ask people to clean out bird boxes by placing the above text on their websites, reminding members to speak to their friends and neighbours and contacting the local media. This may help to reduce the number of "swarms" they are asked to deal with.