Bumblebee count No.2

My timetable and dismal weather during my free time this week prevented me returning to the bumblebee nest until a week after my last count. Not that the weather was inviting as I began walking to the site, the dark clouds suggested rain and the strong wind was cold. Not surprisingly I saw no honeybees, hoverflies or solitary bees about. Bumblebees generally shrug off bad weather, they are tough determined little beasts.

The only other bee seen was this queen Bombus
terrestris (Buff-tailed bumblebee) feeding on
heather near to the nest.

As I approached the nest the first question in my mind was, would it still be active? No one knows with certainty how long these winter colonies last. Within a couple of minutes a bee arrived and entered the nest so I began setting up my tripod and camera to film the nest.

Filming the bees entering or leaving the nest offers a number of advantages. From the perspective of personal comfort it is preferable to watch a video in the warm than stand outside conducting the count. If my comfort is of no concern, it also offers greater accuracy as the video can be replayed and examined frame by frame. Having video also offers the possibility of recounts and the availability of other people to view it.

It had rained earlier in the morning and bumblebees have a tendency not to collect wet pollen. Out of the forty bees that returned to the nest only one carried pollen. It was debatable whether the bee had collected the pollen, it was spread on its hind legs rather than rolled into a ball as usual. It returned in the last few minutes of filming and may indicated that the flower's pollen was drying out and some stuck to the hairs on the bee's leg.

The other notable observation was that one worker exited the nest but rather than flying away, turned round and re-entered the nest - see video.

Compared to last week, there were less bees seen in a similar time span. This may be due to several possible reasons. The colony size may have decreased or possibly the bee's foraging activity may have been less due to the adverse weather, wet pollen or the difference in the time of day between the two observations.

The only certainty gained by comparing both day's counting is that more data is needed. It seems I have a new task for next week's to do list.

Bombus terrestris nest - Falmouth 3rd March 2017 11.14hrs+
P/N? = unable to distinguish whether bee had pollen due to hind legs obscured 

DUR MINS	BEES IN	BEES OUT	POLLEN	NECTAR	P/N ?	TOTAL
10	6	6	0	6	0	12
13	8	7	0	8	0	15
10	5	5	0	5	0	10
11	6	5	0	6	0	11
14	5	6	0	5	0	11
11	7	6	0	7	0	13
7	3	4	1	2	0	7
76	40	39	1	39	0	79

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.