Counting Bumblebees in Winter

After watching buff-tailed bumblebees (Bombus terrestris) visiting flowers for the last three winters in Falmouth I have gained some insights into their behaviour but many questions remain. Discovering a nest a week ago will help extend that knowledge a little further. By counting the bumblebees leaving and returning to the nest it is possible to speculate how many bees are in the nest. 

Friday 24th February 2017 Bombus terrestris nest - Falmouth, UK 
P/N? = unable to distinguish whether bee had pollen due to hind legs obscured

TIME	DUR MINS	BEES IN	BEES OUT	POLLEN	NECTAR	P/N ?	TOTAL
14.36	12	16	10	5	8	3	26
14.49	10	9	9	4	5	0	18
15.01	10	6	7	1	4	1	13
15.12	10	16	9	5	6	5	25
15.26	10	5	6	0	3	2	11
15.40	11	5	4	2	2	1	9
15.54	11	5	6	4	0	1	11
16.06	6	6	0	1	3	2	6
TOTALS	80	68	51	22	31	15	119

The first thing to consider when looking at the numbers is that about 60% of the worker bees in the colony will remain in the nest doing such things as helping care for the young.

A rule of thumb guide is to take the number seen in ten minutes and times that by four. Taking the peak result would give us 104 bees in the colony with about 42 of these involved in foraging. As can be seen the activity decreased over the time of observations illustrating how varied foraging activity can be. Taking an average of the 10+ minute numbers gives a colony size of 66 bees with 26 workers foraging.

As a ballpark figure we have a colony size of 66 to 104 bees from the initial observations.

Edit 17th December 2017 - Having reflected on these figures I feel they are probably too low. Given that there are fewer flowering plants in winter it is likely foraging flights take longer. Also the frequency of bees visiting plants in various locations suggests a larger colony size although the possibility of there being more than one colony must be considered.

The question is only really going to be answered with a spade once a colony has completed its lifecycle and is disused. Examining winter active nests and looking at its cells where the larvae turn into adult bees will not only give accurate numbers but also answer how many males and queens were produced. 

If you are imagining me standing over the nest for eighty minutes counting the bees while examining each to see if they were carry pollen, possibly wearing a deerstalker hat and smoking a pipe you are mistaken. I filmed the nest - the breaks between counts was due to moving the camera and reframing. Counting was done in the warm, watching and replaying the video while drinking copious cups of tea.

Of course I could not resist making another bumblebee video from 80 minutes of footage - don't worry it is only 2 minutes long.