Broxhead – Silver Studded Blue – End of 2020 season survey results
Introduction
I have now performed four years of records for this site, the last three are consecutive and with some degree of procedure in that I have visited the lek area regularly during the flight period of the Silver-studded Blue and followed the same planned route every time, only counting those individuals within a 180-degree field of view looking forwards – anything that flies into view from behind is not counted. Typically it is a walk at pace but unlike other people I have met that walk an official transect, I count anything I see, not just those within a few feet of me. Of course I will double-count to some extent when there are many in and around the lek all at once but I believe this is compensated for by the ones I inevitably miss – especially the later season females who will be low and camouflaged by the heather while egg-laying.
Habitat
Broxhead Common is a small dry lowland heath lying between Kingsley and Bordon in North-East Hampshire and about a mile from the edge of Alice Holt Forest. To the north is Alton and the Chalkhills of the North Downs. Broxhead itself lies on the Upper Greensand and is therefore a very sandy heath. Its specialities other than the SSB’s are Small Copper, Common Heath Moth, Brown Argus, Dartford Warbler, Stonechat, Nightjar, Woodcock, Hobby and the rare Sand Lizard. The lek area for the SSB’s is just the other side of the metal gate on the main lay-by between Kingsley and Bordon. I study this area in particular because I have discovered it is a good predictor of the health of the entire Broxhead colony which in fact spreads out over the entire heath, with pocket colonies dotted here and there with the odd individual flying between – but most staying within a short distance of where they emerged. By focussing my attention on this relatively small patch I can do my transect in about 30 minutes – it would take most of the day to cover the entire heath! This means I can repeat the transect more frequently and thus identify population trends over time. Broxhead Common lies adjacent to two other heaths (Kingsley and Sleaford) but the incidence of SSB’s at those is negligible to none and likely this is due to the lack of the relevant ant species the SSB needs to complete its life-cycle.
History of the study
I did perform a full count of the heath in 2010 without any form of procedure but it took a number of visits each around 2 hours duration over some days around the peak of the flight period. I don’t intend to dwell on this other than to say I was surprised at the healthy overall count of over 550 individuals and by this means identified the more densely populated areas and I discovered where the main lekking site was.
My first proper study was in 2018. Numbers were much the same across the entire reserve as they had been in 2010 (I did a lek count AND a full heath count that year), so presumed at the time this was the “normal” population size. But in 2019 there was a shocking surprise drop in numbers. So what would 2020 bring?
2020
This year we had a long wet winter followed by an exceptionally dry spring. The heath at the end of May was parched dry and many of the lower heather plants had yellowed. It was, I thought, touch and go whether we would get much of a heather bloom and how the butterflies would fare with such a reduction in nectaring opportunities. As the days rolled on the SSB’s started to emerge, males first as usual. The first incidence I saw was on 28th May (2 males). This was my earliest ever record of this species at this site, a clear 9 days earlier than in 2018 and 18 days earlier than in 2019. It was odd to see them flying with so few heather blooms on show. It was very worrying. The dry spell continued and the numbers started to increase along the same curve as they did the previous year only much earlier. We did get the odd spot of rain and thankfully – those heathers that had not died back completely started to bloom. I breathed a sigh of relief. And then, just as the numbers were starting to build nicely the temperature crashed and we got four days of rain with a breeze. After that cleared away I discovered the numbers had been culled back but a further spate of sunny weather corrected that and the rain had actually been a great benefit to the flora with many more heather bushes now blooming. This explains the double peak with the dip between on the 2020 chart below, the previous two years were pretty much consistently fine for the entire flight period by comparison.
Overall, this year the numbers were slightly better than last year suggesting the population not only survived the crash of 2019 but were clawing their way back to the “good times”. Only further annual reviews will be able to establish this for sure. Possibly the species has a boom and bust cycle over years such as some other species do. Whatever it was in 2019 that had such a drastic effect, the males were hit a lot harder than the females, and since it is the number of females that dictate future size of population it should be of no surprise that the colony held up into this year quite well.
The peak of males was 118 on date 26th June.
The peak of females was 24 on date 24th June.
The total population in and around the lek over the entire flight period was therefore
z = 118+24=142
Since my observations in previous years concluded that the lek area typically contains 2/3 of the entire site population, the estimate of the entire population at Broxhead this year is therefore...
Total site population P = Population in and around lek * lek to site ratio
P = z * 3/2 = 213
Last three years trend charts for comparison. Blue bars are males, red bars are females.
The double peak suggests I have undercounted though. Not all of the 113 males I saw in the first peak can be assumed to be included in the 118 males in the second peak because the dip suggests some of those died in the cold snap.
Post-script: Other sightings for Broxhead this year.
During this season I saw a female SSB with wobbly wings, obviously not yet fully pumped-up after newly emerging and yet already she was vigorously wing-rolling, albeit in an odd “wings-ajar” kind-of way. Pictures of this can be found under the “General” forum entry for wing-rolling.
Plus my very first male Sand Lizard in fully resplendent mating colours. As I approached – it did not move. Then I realised it was dead. Sad. It reminded me of Monty Python’s Parrot sketch. Beautiful “plumage” but dead. Does thinking that make me a bit weird? Probably. My wife (who I was out walking with at the time), moved it off the path and covered it with foliage, giving it a decent “burial”.
The birds were very good this year. I stood watching two Dartford Warblers (a pair) for ages, taking pictures. They went to ground for awhile then suddenly appeared off to my side. How on earth did they do that I wondered. Then the two I had been watching re-appeared in front of me again. Oh, there were TWO pairs! Marvelous.
Oh, yes and the Stonechats too…