I had a winter's walk to my apiary yesterday afternoon and after giving them a way out (the entrance was covered in snow), shortly afterwards I found a few apparently dead bees on the snow in front of the hive. I picked up one and warmed it up with my breath, and it starting moving again.
I later was at a house where a large colony lives in the roof. We found tens or even hundreds of dead bees on the snow below their entrance. Again I was able to revive one by breathing on it.
I assume these are bees that were going out to void themselves and then got too chilled before they returned to the hive. Anyone have any similar experience or a different explanation?