Black bees behaving strangely

Observation - Black bees behaving strangely. Description: I have watched, so far, about 10 black bumblebees with orange bottoms. They are crawling around slowly on a patch of grass in the sun. They then seem to keel over and die. I have gently picked up a few and put them on a flower bed, but they are still dy

I have watched, so far, about 10 black bumblebees with orange bottoms. They are crawling around slowly on a patch of grass in the sun. They then seem to keel over and die. I have gently picked up a few and put them on a flower bed, but they are still dying. Is this the end of their season, and why are they crawling on the grass? Help, please. I do have a short film of it, but cannot work out how to upload it from my iPad.