The National Trust's Large Blue blog

Chrrsitine Tansey, our extremely capable and friendly volunter ranger at Collard this year has produced a report on the season at collard Hill in 2011. It is very thorough and readable and we thought it would be interesting for all collard enthusiasts to see. As well as the report itself Christine has undertaken the mamouth task of inputing all the data we capture each year – transect figures for all butterlfy species, weather, visitor numbers etc. This was a real labour of love and hasd taken hours so, on behalf of all those interested on the fortunes of wildlife and people at Collard, thank you Christine.

Here is her report as a pdf file:  Large Blue Report 2011 – Updated

This file can take some time to load so here is her summary (for graphs see pdf file)

1.0 Executive Summary:

 General 

• 2011 was another successful year at Collard Hill, despite some challenging weather conditions, the trend of increasing butterfly and visitor numbers continued during the flight season.
• The flight 2011 flight season was the longest ever recorded for Collard Hill, lasting at least 45 days.
• June weather was largely wet and windy; average maximum temperature recorded at Yeovilton weather station was 18.9°C, lower than the June average of 19.3°C for the site. There were also fewer hours of sunshine and over 20mm more rainfall than the June mean.
•  Large Blues egg numbers saw an increase of 28% from the 2010 egg survey; from 30829 to 39602. This corresponds to an increase in the estimated number of adults from 1233 in 2010 to 1584 in 2011.
• Myrmica sabuleti ants continue to be well distributed across Collard Hill, and the drought conditions experienced during 2010 and early 2011 appear to have had a limited impact upon their presence.
• Collard Hill received good publicity both nationally and locally in 2011, and despite the weather, the number of visitors saw a slight increase on 2010, up 1.4% from 1163 to 1179.

 Emergence 

• The first Large Blue of the season was seen by David Simcox on the 27^th May 2011, and estimated to have emerged on 25^th May. David Simcox also saw the final butterfly on 10th July 2011.
• Cattle grazing was removed from the site slightly earlier than in some years. Due to the drought stressed slopes, a lighter grazing pressure was required immediately prior to the Large Blue flight season to ensure that the sward and wild thyme had the best chance of being in good condition.
• The pattern of Large Blues recorded over the flight season is likely to have been influenced by the weather conditions, as there is no clearly discernable increase in emergence leading to a peak flight period (Figure 1). Instead the butterflies seem to have fluctuated over several weeks. This may have arisen through a combination of factors acting on the butterflies such as an extended individual life span in poor weather and the early emergence stimulated by good weather.

Figure 1: Daily totals of Large Blue butterflies recorded on and off transect.

• The 2011 season experienced strikingly different weather conditions to 2010, with a period of drought through April and May giving way to rain and high winds dominating in June and early July. Weather is likely to be a major contributing factor to the differences observed by transects; on a day-to-day basis and in the patterns of Large Blue emergence over recent years.
• The increase in egg numbers, found on a survey conducted by David Simcox, Jeremy Thomas and Sarah Meredith, show that although 2011 experienced a decrease in Large Blue numbers recorded on transects, adult numbers continued to increase.
• The length of the flight season, influenced by individuals living longer, an early emergence, and a long tail off period, may also be a consequence of the weather conditions.

 Visitors 

• Weekends saw 43.4% of total visitor numbers, averaging over 100 visitors per weekend. Weekdays had an average of 27 visitors per day over the season.
• Of visitors surveyed, 27.5% were classified as ‘Local’, this was taken to be an area roughly comprising between Bridgewater, Wells, Yeovil and Gillingham.
• The proportion of visitors from the local area and the broader South West and West region was 65.8%, up from the 63% recorded from Wessex, Devon and Cornwall in 2009.
• Once again, Large Blue numbers exceeded the number of visitors to the site.

2.0 The Large Blue: 

The 2011 flight season was a challenging but overall successful one for the Large Blue at Collard Hill. The prospects for the season were mixed after the drought conditions yet record-breaking numbers recorded in 2010. In addition, after another snowy winter, the start of 2011 saw warmth arrive early, with above average maximum temperatures from February until March. This contributed to many butterfly species emerging early across the UK, in some cases flying 2-3 weeks before expected. Butterfly Conservation reported 2011 as one of the earliest years on record.

 Drought conditions were also experienced in April this year, with only 5.8mm rain falling in comparison to the monthly average of 47.3mm. Rainfall remained low in May; 36.6mm of rain was recorded, well below the average of 48.9mm. These conditions led to an earlier removal of stock from Collard’s slopes, when a suitably short sward had been achieved. There were concerns that the site condition was on a tipping point of being too drought stressed, which could have affected the ant colonies and Wild Thyme. Sarah Meredith recorded some signs of stress on Thyme plants across the site, during a phenology study for the University of Oxford.

 By the first week of June, the weather had broken, and the subsequent rain helped restore the slopes. By the end of the flight season much of the Thyme had recovered enough to flower, providing enough of the required host plant for the Large Blues. June temperatures were only 0.4°C lower than average, however hours of sunshine were 12.6 hours lower than average. In contrast, rainfall recorded was 71mm, well over the June average of 57.2mm. Average wind speed data is not available for the flight season, but it is clear from the experience of volunteers on the hill, that there were consistently high winds at Collard Hill during June.

 Apart from the weather conditions, an optimal sward height and structure was achieved by the grazing regime on site prior to flight season. The success of grazing management at Collard Hill has been marked since 2005, when the adoption of Red Devon cattle and ponies led to continued improvement in site condition. In 2011, that the sward was grazed very well is likely to have been particularly important, given the weather conditions were not ideal.

 Large Blue emergence at Collard Hill in 2011 was the earliest ever recorded, as it is likely to have started on 25^th May, with the first sighting by David Simcox on 27^th May. Although their early arrival was unprecedented at Collard Hill, Large Blue emergence times vary significantly from site to site, even within the Polden Hills. The very warm spring weather, in addition to other factors, may have contributed to this early appearance.

 Table 1 shows the emergence pattern of Large Blues at Collard Hill since their reintroduction. 2011 has proved exceptional for several reasons; it recorded the earliest emergence of the butterflies by at least a week, the flight season lasted at least 45 days, and it did not reach a clear peak flight period. Peak numbers of Large Blues were recorded on two days a week apart; the 14^th and 21^st June. This produced a season without a very clearly defined build up and tail off of butterfly numbers, which is illustrated by Figure 1. By the end of the season numbers did drop dramatically, with only a few individuals sighted after 3^rd July.

 Table 1: Emergence of the Large Blue at Collard Hill.

* Year larvae and adults reintroduced to site

** No data available

*** First sighting, but estimated emergence date 25^th May 2011

The change in emergence pattern from previous years is likely to have been influenced by several factors. The role of weather is probably an important one, with consistently poor conditions lengthening the lifespan of individual butterflies, and therefore the flight season. In addition to this, the simple size of the population at Collard Hill may have elongated the flight season. As butterfly and egg numbers have increased, a more staggered emergence will naturally occur.

Although there appears to have been a less clear cut peak flight period during 2011, Figure 2 does show the general trend of increasing and tailing off numbers. What are also apparent, are the gaps in the transect record this season, which occurred when weather conditions were too poor to conduct transects. There were several days when only one transect was walked, due to inclement weather over a part of the day. Additionally, the first emergence of Large Blues occurred around a week before recording began, thus the initial increase in numbers is not seen from the data collected. All these issues contributed to the incomplete record of the season.

 Figure 2: Number of Large Blues recorded on and off combined AM and PM transects during the flight season.

2.1 Large Blue Egg Count

 As in previous years, daily transects were walked at 11am and 2pm, along the route seen in Appendix 7.1, during the flight season at Collard Hill. While unable to provide an accurate estimate of population, transects continue to indicate the use of the site and distribution of Large Blues across it. In 2011 in particular, weather conditions were frequently poor, so transects recorded far fewer butterflies than were present. Transect under-recording was confirmed by the egg survey conducted in July by David Simcox, Jeremy Thomas and Sarah Meredith. Using the egg counts, the estimated number of adults present is calculated by assuming each female lays 50 eggs, and that females make up 50% of the population. This method gives an estimate of 1584 adult Large Blues flying in 2011. In contrast, if it is assumed that four butterflies are present for each adult seen during the daily transects, the total estimate is only 1056 individuals, thus illustrating the inaccuracy of that method for providing population estimates.

 Despite initial uncertainty as to the effects of the dry 2010 season, continued drought in spring 2011, and inclement weather during the 2011 flight season; Large Blue egg and estimated adult numbers saw a sizeable increase. Table 2 summarises the changes in egg and adult numbers since the year of reintroduction. Figure 3 illustrates the pattern of increasing egg numbers in recent years.

 Table 2: Large Blue egg numbers and estimated adult population at Collard Hill.

 Figure 3: Large Blue egg numbers recorded at Collard Hill since their reintroduction.

 Dips in the numbers observed in 2002 and 2005 reflect years during which undergrazing and weather had significant impacts upon the Large Blue’s success. Since 2005, there has been a consistent increase in Large Blue numbers. As egg numbers have increased, so has the likelihood of a greater emergence in the following flight season. Since 2008, when the numbers observed almost doubled, there have been large increases and 2011 saw a 28% increase in egg numbers from 2010. Despite the uncertainty over weather conditions in 2011, the upward trend has continued at a similar level to the 30% increase between 2009 and 2010. Different areas of the site are likely to have been colonised by the Large Blues as they have come into favourable condition.