The Tree Bumblebee (Bombus hypnorum) is a common bumblebee species in continental Europe and northern Asia.
The thorax is usually of a uniformly ginger colour (but examples with a darker, or even black thorax occur), the abdomen is black haired and the tail always white.
The females (worker bees) are often (but not always) small, males (drones) are much bigger, and the queen varies in size.
It prefers to build its nest above ground and often inhabits bird boxes.

In my yard, Hilversum, The Netherlands, 24 June 2013