Bumblebees don't form swarms like honeybees do. But you might see a few bees flying outside a nest, especially if it's in a bird box or somewhere else above ground. These are male bees hovering outside the nest, waiting for queens to emerge so that they can mate. Male bumblebees cannot sting, and they pose no risk to you at all. Therefore, no action is needed to be taken for them. There is more information on Tree bumblebees in this article (pdf 1mb).
You can see a bumblebee drone cloud in the video below:
“Bumblebees are lovely little creatures - their bright stripes and gentle buzz bring colour and sound to our summer gardens. They are also very important because they pollinate our wildflowers and crops. Sadly things aren't going well and some species are threatened with extinction.
I'm really concerned by these declines and I'm pleased to support the work of the Bumblebee Conservation Trust."