A population comparison between two UK Glow Worm colonies 2013
2013 provided us with the opportunity to make a direct comparison of the Glow Worm colonies at different sites within the UK. We were primarily looking at comparing the pattern and rates of female emergence at the two colonies during a complete season. This would obviously mean that both sets of data, produced by the same recording methods, would have to be applied by recorders working each site.
Such comparison work, had long been an aspect of Glow Worm research that Martin Dale and ourselves had discussed for several years, so we would like to thank Jenni Cox for providing us with this exciting opportunity and allowing us to use her meticulous data and photographs. Her recording work and considerable effort is enormously appreciated.

Our survey work and methods of recording Glow Worms at Clipstone Old Quarter in Nottinghamshire, is regarded as being some of the most thorough, but it was impractical to expect Jenni to mark each female in the way we do, as part of the methodology we employ at Clipstone Old Quarter.

For the aims of this particular research, female marking/labelling etc was not required anyway. All that was needed was the total number of females within a given area, on every night throughout the entire season.

Many people reading this will have no idea of the effort required for such a task, so for us to find someone willing to undertake this for potentially (depending on season length) a hundred or more consecutive nights, was all the more remarkable.

Habitat extremes - a brief introduction to two very different Glow Worm habitats

The right and upper right photographs, show part of Jenni's recording area on the Great Orme, with our own recording area of Clipstone Old Quarter in the two photographs below.

Jenni's Glow Worm study site is on the north-west coast of Wales on the Great Orme at Llandudno, Conwy and ours at Clipstone Old Quarter, an area of commercial forestry within Sherwood Forest, Nottinghamshire. Due to the expanse of the Great Orme, Jenni worked a much smaller area of the site each night (the comparison area), which she was able to cover easily.

Having such different sites was especially interesting, due to the extreme differences shown by the two. The sites differ not only in habitat, but also climatically, with the coastal site of the Great Orme being noticeably warmer by several degrees, evident in the temperature data sent to us by Jenni. If these two sites produced any similarities in the returned data, then that would mean most UK sites would be likely do the same.
Comparison data on male and female Glow Worm emergence

Both sites also show huge differences in their respective Glow Worm populations. Whilst we are confident that Clipstone Old Quarter is one of the best inland colonies (its certainly the most consistantly recorded colony in the UK) our record single count of females is still below 100. Over the past few years, counts of many hundreds of female Glow Worms have been recorded by Jenni on the Great Orme and Robin Scagell of the UK Glow Worm Survey also counted over a hundred females on his visit there last year.

The resulting data proved to be especially interesting. Immediately obvious from the two graphs produced below, is the huge difference in the numbers of Glow Worms between the sites. Because of the higher Glow Worm numbers at the Great Orme, rises and falls are more evident in Fig 02. Rises in numbers are of course, caused by the sudden emergences of females. Falls are due to numbers of females being mated the night before, following a large emergence of males and females subsequently not glowing again.

At a glance, there may seem to be no comparison in the data at all, but there were five key points of rise and fall in numbers during the course of the season. Ignore the number of dates between the key points in Fig 01 and Fig 02, as for this study we were looking for the peaks and not the length of time between them.

Fig 01. .. Seasonal site counts and female Glow Worm emergence/numbers at Clipstone Old Quarter in 2013
Fig 02. .. Seasonal site counts and female Glow Worm emergence/numbers at Great Orme in 2013
Key points 1 and 2 both represent small increases above the general increase in female Glow Worm numbers, occurring before a sudden rise to peak numbers (season maximum) at key point 3. As the number of females begin to decline, key point 4 marks another slight increase against the general decline in females. Key point 5 marks the second of the two post-maximum peaks in female numbers.

The number of males at the Great Orme was astonishing and reached three figure counts on six occasions. Because of the sheer number of female Glow Worms on site, Jenni rarely had the opportunity to check for males with females. Instead, the extraordinarily high numbers of males, were largely to be found on the pavement below street lighting, situated at the end of Marine Drive near West Shore. Peak male numbers, interestingly occurred just after the peak numbers of females.

As far as we are aware, the number of times a male can successfully mate is still unknown. Possibly these are males that have already mated with females and are now largely spent. But could previously mated males be out-competed by freshly emerged, more vigorous males?, or is the attraction is just down to street lighting?

Fig 03. .. Seasonal site counts and male Glow Worm emergence/numbers at Great Orme in 2013
Despite the males congregating under street lighting, it is clear from the number around the Great Orme, that females must be successfully attracting males, although there are fewer females in the general area of the street lights. There is ample opportunity for further study on this subject, which could involve the marking of all males found under street lighting, then checking for females mating with marked males over the following nights.
Colour coding the males, using a dfferent colour each night, may allow more detailed and thorough analysis. Alternatively, males found with females could be marked to see if they eventually turn up under street lighting.

Fig 03 shows the seasonal site counts and male Glow Worm emergence and numbers at the Great Orme in 2013. We have superimposed the five key points of female emergence/peaks from Fig 01 and Fig 02, against the number of males each night on the Great Orme.

This proved interesting, as it showed that the peak number of males under street lighting, actually occurred just after the high seasonal peak numbers of females.

There is a small peak which coincided with peak numbers of females (key point 3) so does this mean that males were successfully finding and mating with females, then more greatly attracted to the street lights on the nights following mating? This could account for the slightly later peak in the number of males.

Table 01. ... Total number and percentage of females glowing for one evening at Clipstone Old Quarter during 2013
Inclusive dates based on first appearance of numbered females.The total of females involved is shown in column three and the percentage of 'one night' females in column four.
Female 001-050 Jun 08 - Jun 23 20 40.0%
Female 051-100 Jun 23 - Jun 29 34 68.0%
Female 101-150 Jun 29 - Jul 02 31 62.0%
Female 151-200 Jul 02 - Jul 05 29 57.9%
Female 201-250 Jul 05 - Jul 06 24 48.0%
Female 251-300 Jul 06 - Jul 07 30 60.0%
Female 301-350 Jul 07 - Jul 08 28 56.0%
Female 351-400 Jul 08 - Jul 09 33 66.0%
Female 401-450 Jul 09 - Jul 12 33 66.0%
Female 451-495 Jul 12 - Jul 26 25 55.5%
It is very tempting to jump to the conclusion that street lighting along Marine Drive could be having (or has had) an adverse effect on the Great Orme Glow Worm population. But without successive years of accurate data, such a conclusion cannot really be reached at this stage, but there is no reason why we cannot speculate or propose theories.

Although the number of males recorded at Clipstone Old Quarter never got near double figures, it was obvious through the high percentage of 'one night' females shown in Table 01, that male numbers were particularly high in 2013. Going purely by male counts, the peak dates for males at Jenni's Great Orme site was between June 3rd and 8th, with a maximum of 268 males on June 7th. Peak numbers of males at Clipstone Old Quarter came a few days earlier, between June 23rd and 29th.

Table 02. .. Comparison site and Glow Worm data 2013
TC = Site temperature .... Nm = Total males recorded .... Nf = Total females recorded
    Clipstone Old Quarter   The Great Orme
Date   Weather conditions / summary   TC Nm Nf   Weather conditions / summary   TC Nm Nf
.... . . . . . ..... . ..................................................................................... . . . .....
05/06/13   Cloudy and mild   12C 2 0   Dry   14C 0 0
06/06/13   Clear and fairly mild   09C 0 0   Dry   15C 0 1
07/06/13   Clear and mild   10C 1 0   Dry and clear   15C 0 1
08/06/13   Cloudy and mild   10C 0 2   Dry and clear   16C 0 6
09/06/13   Cloudy and mild   12C 0 4   Dry and clear   15C 0 14
10/06/13   Clear and cool   08C 0 1   Dry and clear   17C 0 23
11/06/13   Cloudy and very mild   14C 0 6   Cloudy with showers, but mild   17C 1 29
12/06/13   Cloudy and very mild   15C 1 6   Windy, with some rain during the day   16C 0 28
13/06/13   Cloudy and mild   11C 0 2   Windy, with some rain during the day   14C 0 22
14/06/13   Cloudy and mild with rain   13C 0 6   Overcast with rain   14C 0 16
15/06/13   Partly cloudy and mild with a little light rain   12C 0 9   Windy, with some rain during the day   14C 0 48
16/06/13   Cloudy and mild   13C 3 14   Overcast with showers   15C 0 37
17/06/13   Cloudy and mild   13C 0 7   Overcast with light rain and breezy   17C 0 63
18/06/13   Cloudy and warm   16C 0 6   Cloudy and warm   18C 50 54
19/06/13   Partly cloudy and mild   14C 2 6   Clear and very mild   16C 29 25
20/06/13   Heavy rain late afternoon, continuing into the night   13C 0 9   Cloudy, with some rain during the day   17C 52 35
21/06/13   Clear and dry, becoming overcast   15C 4 10   Overcast with showers and breezy, but mild   15C 44 75
22/06/13   Clear and dry, with a full moon   13C 0 6   Overcast with showers and breezy   14C 15 62
23/06/13   Cloudy and mild, breezy   12C 1 18   Damp, overcast and windy   13C 0 43
24/06/13   Clear and cool   08C 0 6   Damp, overcast and breezy, but mild   15C 15 104
25/06/13   Cloudy and mild   12C 1 22   Overcast and breezy   16C 38 159
26/06/13   Cloudy and mild   14C 7 18   Clear skies and breezy   15C 13 218
27/06/13   Cloudy and mild, some rain during the day   12C 1 15   Damp ground, breezy with fog   14C 21 212
28/06/13   Dry, cloudy and mild, rain during the day   12C 1 6   Cloudy and breezy   15C 34 153
29/06/13   Cloudy and mild, becoming clear later   13C 4 15   Clear and breezy   18C 44 192
30/06/13   Cloudy and warm   16C 2 23   Cloudy and damp, breezy   15C 73 204
01/07/13   Clear and cool   09C 0 8   Cloudy and windy   16C 45 169
02/07/13   Cloudy and warm, with some light rain   15C 0 39   Overcast wth showers and windy   17C 28 190
03/07/13   Clear and mild   12C 3 29   Clear and breezy   16C 103 192
04/07/13   Partly cloudy, becoming clear   13C 1 32   Clear and breezy   17C 134 154
05/07/13   Clear and warm   17C 2 78   Cloudy with a light breeze   18C 116 123
06/07/13   Clear and warm   16C 1 81   Clear skies with a light breeze   18C 149 111
07/07/13   Overcast and warm   19C 4 90   Clear skies with a light breeze   20C 268 113
08/07/13   Clear and mild   13C 3 46   Clear and warm   21C 209 120
09/07/13   Partly overcast and warm   20C 4 81   Clear and warm   21C 77 161
10/07/13   Cloudy and mild   13C 1 52   Cloudy and breezy   20C 94 109
11/07/13   Clear and mild   12C 1 36   Clear and breezy   19C 76 110
12/07/13   Cloudy and very warm   21C 2 57   Clear and very warm   21C 51 100
13/07/13   Clear and very warm   18C 1 30   Clear and breezy   18C 30 77
14/07/13   Clear and warm   14C 0 18   Clear and warm   18C 29 75
15/07/13   Clear and very warm   19C 1 20   Clear and warm   18C 44 80
16/07/13   Clear and very warm   21C 0 21   Clear and warm   18C 19 71
17/07/13   Clear and very warm   19C 0 16   Clear and very warm   21C 24 67
18/07/13   Clear and very warm   19C 1 12   Clear and very warm   22C 9 67
19/07/13   Partly cloudy and very warm   19C 0 10   Clear and very warm   23C 15 44
20/07/13   Cloudy and warm   18C 0 10   Cloudy and breezy   22C 11 15
21/07/13   Clear and warm   16C 1 9   Clear and breezy   21C 1 16
22/07/13   Cloudy and warm with thunderstorms   20C 0 6   Cloudy and misty, with a slight breeze   22C 0 8
23/07/13   Clear, warm and misty   19C 0 5   Clear with a light breeze   20C 1 7
24/07/13   Cloudy and very mild   20C 0 6   Cloudy, some rain and a light breeze.   22C 0 2
25/07/13   Cloudy and very mild   20C 0 4   Cloudy and breezy   21C 0 4
26/07/13   Clear and mild   18C 0 3   Clear   19C 0 5
27/07/13   Heavy rain   17C 0 1   Overcast with rain   20C 0 3
28/07/13   Dry, rain earlier in the afternoon   18C 0 0   Cloudy with showers   19C 0 0
29/07/13   Clear and mild   16C 0 0   Cloudy and windy   18C 0 2
30/07/13   Clear and mild   18C 0 0   Cloudy   18C 0 0
31/07/13   -   - - -   Cloudy with showers and breezy   20C 0 2
01/08/13   -   - - -   Clear and breezy   19C 0 1
02/08/13   -   - - -   Clear and windy   20C 0 1
03/08/13   -   - - -   Cloudy and breezy   19C 0 0
04/08/13   -   - - -   Cloudy and windy   20C 0 0
Table 02 shows the general data for both sites during the glowing season. The temperature range is interesting, clearly showing the more mild conditions on the coast, with ranges of between 8-21C at Clipstone Old Quarter, compared to 13-23C at the Great Orme. The length of the female glowing season was also very similar, lasting for approximately 50 nights at Clipstone Old Quarter, compared to 58 nights on the Great Orme.
Geographical and meteorological data
Great Orme ... Llandudno Conwy
Location: ... 53 19' 50.27" N ..... 3 51' 54.68" W .....
Elevation above sea level: ... 51m
Minimum and maximum site temperatures: ... 13C and 23C
Temperature variation: ... 10C
Clipstone Old Quarter ... Edwinstowe Nottinghamshire
Location: ... 53 11' 48.92" N ..... 1 05' 37.10" W .....
Elevation above sea level: ... 86m
Minimum and maximum temperatures: ... 8C and 21C
Temperature variation: ... 13C
Site distance apart
Summary and the possibilities for future comparison study

It would help if similar comparisons between Clipstone Old Quarter and the Great Orme, possibly incorporating Glow Worm sites in the south of the UK or Scotland, could be made in the coming years. There's plenty of possible research options, as it is becoming clear to us, that we still actually know very little about the intricacies of Glow Worm biology.

Clipstone Old Quarter Glow Worm summary 2013