A number of FOLK volunteers undertake butterfly surveys on the Hill and Common. The reasons for undertaking such surveys are given below, followed by a report on results of the survey work in 2015 and 2014.
Following entry into Higher Level Stewardship scheme and the change to “extensive” grazing on Charlton Kings Common, it was considered to be important to monitor changes to the flora and fauna to ensure that the management now being applied to the site is improving rather than degrading the habitat. To this end a botanical survey was instigated in 2012 and the following year, 2013, a butterfly monitoring scheme was introduced to complement this.
To establish what (diversity and abundance), where (location with reference to habitat type) and when (flight periods) butterfly species are present across easily accessible areas of the whole site over a number of years.
The area has been divided into 4 for the purposes of the survey:
- Leckhampton Hill
- Brownstone Quarry
- Charlton Kings Common to the west of Windass Hill, and
- Charlton Kings Common east including the Cowslip field.
Each area has been subdivided into sections, roughly equating to habitat type, through which a fixed route is walked by volunteers. For each section, the butterflies seen close to the recorder (within 2.5 metres) are recorded and counted by species. Surveys are done between April and September, every week or two, but dependent on there being suitable warm and/or sunny weather conditions.
The surveys so far have established where on the site adult butterflies of each species can be found. This has provided guidance as to areas that would benefit from particular habitat management, and corridors are being opened up to ease movement of species across more of the site.
Summaries of the surveys undertaken in 2016, 2015 and 2014 are included below.
Results of the Survey in 2016
The total number of surveys done this year was 70, on 56 different days, giving an indication of the scarcity of suitable opportunities during the rather poor summer in general. Five of them resulted in no sightings whatsoever.
As a whole the season was later than 2014 and 2015, but earlier than 2013. March was particularly cold and wet and with the last frost being recorded on 30th April, it was no surprise that only 13 butterflies had been recorded by the end of that month. After an exceedingly wet June, during which there was a succession of torrential rain storms, often accompanied by thunder and lightning, conditions in July improved markedly. This corresponded with a surge in butterfly numbers boosted as they were by the emergence of the common grassland species (Ringlet, Meadow Brown and Marbled White).
Abundance of butterflies, both on the best day and during the flight period, were the lowest recorded for about half the species, despite the addition of two new survey sections. Some species numbers reduced by as much 90% from the peak recorded in 2014, but most were down by 30% to 50%. Green Hairstreak and Dark Green Fritillary fared worst, though these have never been abundant or widespread over the whole site. Of the grassland specialists, Common Blue numbers reduced by about 60%, and Meadow Brown and Small Heath were down by 50%, though Ringlet were less affected. A late surge in Speckled Wood sightings enabled them to buck the trend, recording the highest numbers in 2016 over the years of the survey period.
Once again, there appeared to be a further contraction in the distribution range of a number of species. Common Blue, Small Heath, Dark Green Fritillary, Peacock, Small Tortoiseshell and Comma seemed to be the most affected. The last three all hibernate as adults over winter, and seemingly had a poor survival rate as few Small Tortoiseshell and Peacocks were seen in the spring and no Commas. By contrast, the number of different species seen in each section has remained more constant where there is limited active management, with the wider variations occurring in the grazed parts of Charlton Kings Common.
As a good indicator species for unimproved limestone grassland, the distribution and abundance of Common Blue over the whole site for each year the surveys have been undertaken has been analysed. This has identified the sections where the butterfly has been regularly recorded, so is in all likelihood breeding in those areas provided the larval food plant (Bird’s-foot Trefoil) is present. Interestingly, where it is most abundant, there is generally little or no grazing.
In total 3589 butterflies were identified and recorded between 13 April and 8 September 2016 (4580 in 2015, 6902 in 2014 and 3129 in 2013). The variation over the years (shown in the chart below) most probably reflects poor weather, as periods of torrential rain can decimate adult populations, (numbers take a while to build up again from later emergences) or cloudy, cool and/or windy conditions that are unsuitable for surveying (butterflies will only fly of their volition in sunny or warm conditions).
The addition of a new survey area, known as Hopkins field, proved a boost to the overall figures in 2016, in so far as it had a reasonable number and variety of species, despite the lack of management (other than the occasional mow to reduce the scrub encroachment) over the last few years. Sightings included both Dark Green Fritillary, Silver-washed Fritillary, Common Blue, Brown Argus, Dingy Skipper and Marbled White.
Whilst the results of the survey indicate a reduction in butterfly numbers (despite the increased survey area), the decrease is unlikely to be caused solely by any particular management action (ash, scrub and gorse clearance) as other sites in the Cotswold on which regular monitoring is done showed similar declines and variations in 2016.
However, grazing pressure, especially during peak flowering time (June and July) may well be having a detrimental effect, made worse by the north and easterly aspect of Charlton Kings Common and the generally less herb rich sward due to the dominance of Tor grass of which the cattle are not particularly fond. Action to remedy this situation is now being taken as more land owned by Cheltenham Borough Council adjacent to the common (Hopkins field mentioned above) has been fenced and will be used to reduce grazing pressure on Charlton Kings Common at times. Over the next couple of years it will be interesting to see whether and how butterfly numbers respond to this initiative, both on the Common and in Hopkins field itself. In the meanwhile, with some cold winter weather (this appears to be beneficial to the survival of the early stages of butterfly development) and the anticipation of a warm and sunny season to come, we can look forward to another season.
Results of the Survey in 2015
Although there was a slightly earlier start to the season as hibernators (Small Tortoiseshell, Peacock and Comma) emerged on a warm bright day at the beginning of April, later the weather often failed to play its part helping to make surveying a rather frustrating occupation. Otherwise the flight periods of most species followed in a similar pattern to 2014 and earlier than 2013.
Peak populations (the highest number counted on a single day) of Common Blues, Large White, Meadow Brown, Brown Argus, Small Tortoiseshell, Speckled Wood, Gatekeeper and Small Heath were the highest recorded so far (in 3 years) though this was more likely to reflect the generally poor weather which encouraged all the recorders out on the one and only suitable day of the week than an actual increase in butterflies!
Only Common Blue exceeded the previous highest total recorded in a season. Overall numbers were lower in 2015 (4580) than 2014 (6902). A direct comparison with 2013 (3129) is not possible due to a change in the method of recording.
Of more concern is the possible contraction in the range of some species across the site, even though the habitat remains suitable for them. Dingy Skipper was only recorded in 9 sections (2104 – 16), Small/Essex Skipper 17 (25), Orange Tip 4 (18), Small Copper 1 (3), Dark Green Fritillary 9 (13), Marbled White 27 (30), Gatekeeper 25 (29), Ringlet 27 (30), Small Heath 24 (27) and Duke of Burgundy 2 (3).
No Wall were reported either from surveys or casual sightings, and this is indicative of its decline at all of its previously known inland sites.
The Leckhampton Hill side of the commons continued to have the greatest number of species present and seemingly the most stable populations with minimal fluctuations recorded over the 3 years (+ or – 3 species).
In conclusion, the numbers of butterflies recorded were down on 2014, but generally the winners and losers were in line with local trends when compared to other regular survey work done in the Cotswolds in 2015. However, it was evident that the number of flowers providing nectar sources to the adult butterflies has reduced though a combination of factors: the weeds of disturbed ground that were so abundant following the fencing and walling work were less common in those areas and the number of cattle grazing has increased, thereby increasing the amount of vegetation, and with it the flower heads, consumed and therefore depriving the butterflies of valuable sources of nectar and larval food. What effect this has or will have on the success or otherwise of breeding populations on site it is too early to say.
Results of the Survey in 2014
After the slow and late start in 2013, the 2nd year of regular monitoring began early. The weather encouraged all the recorders out and about from the first week in April, and the hibernators (Brimstones and Peacocks) were already flying. On 16th April, the first Green hairstreaks and Orange tips were sighted, almost 3 weeks earlier than 2013.
As the weather continued fine and warm, Dingy skippers were flying by 3rd May (which was just more than 3 weeks earlier) and by mid-month were recorded in good numbers, along with Common blue and Small heath. The trend of early emergence compared with last year continued with most of the remaining species, but with a corresponding early last sighting date, with few if any Marbled Whites and Ringlets surviving beyond the end of July, when last year they were seen until the end of August.
The highlights of the season must be the sighting of Silver-washed fritillary and Adonis blue, the former initially not on the survey route. However, it was subsequently seen on various days by 3 different recorders on the Leckhampton Hill section just above the Bridge car park on Daisybank Road, with a minimum of 4 individuals present. Whether the single Adonis blue was there naturally or by introduction cannot be determined. It has in the past been recorded spasmodically across the site, but never appeared to successfully establish a viable colony.
The Duke of Burgundy was again recorded on Charlton Kings Common, mainly in the area above the golf course. At the height of the flight period, a timed count was done by a member of Butterfly Conservation, during which a total of about 30 were seen, which was an excellent result, even though some may have been counted more than once.
A disappointingly small number of Wall browns were recorded during surveys, though some were seen by me during casual walks around the hill, so it is still present. After a bumper year in 2013, Dark green fritillaries were not quite so numerous, but were still seen in most previous locations, as well as some new ones. Numbers of Chalkhill blues were also lower, and this was in line with trends on other Cotswold sites.
A single Small (Little) Blue was seen in a new location, but as none of the larval (caterpillars) food plant is known from there, it was presumably what is known as a wanderer, having flown in from elsewhere. Migrant species were more common this year, with several records of Painted ladies and Red admirals.
As not all the records have been received and analysed, just the highlights of the season are recalled here. The comparison of population numbers between the two years will not be able to be made due to a change in the method of recording. But the impression is that there have been some winners and some losers – Marbled Whites up but Gatekeepers down. The reasons for this cannot be determined but are likely to be many and varied such as weather or management related.