These melanistic Tree Bumblebees took up residence in a bird nestbox -- not unusual except that the nestbox was at ground level on a doorstep waiting to be repaired and re-installed. Some had only very sparse orange hair on their thorax and some appeared wholly black except for the white tail.
From time to time larger bees with some red on the thorax would carry a dead melanistic colleague out of the box and lay her gently on the ground.
This continued for several days. The colony left or died out at the end of the 2013 season but the same or a new one is in residence again in 2014.