The Big Butterfly Count 2024 in the Sid Valley

Introduction

The annual Big Butterfly Count is a huge citizen science project run by Butterfly Conservation every year for the last 15 years.  In late July and early August, volunteers find a sunny spot and count how many butterflies and day flying moths (I will just say butterflies from now on for brevity) they see in a 15 minute period across a range of habitats including gardens, parks, fields and woods. 

The project only records from a target list of 20 species that are found across the country and are relatively easy to identify.

This year 80,000 volunteers carried out over 130,000 counts recording nearly one million butterflies, the sheer scale of the project yields data that can be taken as a reliable indication of how our butterfly populations are faring, see Appendix 1 for the latest rankings.  The Sid Valley has an active citizen science population who carried out nearly 200 counts, a small number for reliable statistical analysis but a basis to make useful comments.  This report compares 2024 with 2023 but fluctuations in numbers year on year are the norm, long term trends are more important.  Nationally, 2023 was better than the previous four years and so comparisons may or may not be significant.  Another factor is that this year’s local counts had a higher proportion of meadow sites than 2023 and this will have favoured meadow species in the overall figures.  It is hoped that full local data from previous years will become available soon for a further report.

As the data from year to year are not directly comparable, there are different numbers of counts and differences in the habitats visited, comparing gross figures can be misleading.  To make comparisons more valid, this report focuses on the number of each species seen per count rather than raw numbers. 

Local Summary

Overall, we see more butterflies in our valley than the national mean, 12.6 per 15 minute count compared with the national mean of 7.1. 

The total numbers per count of the 20 species seen locally were down from the 2023 mean of 14.5 per count, a drop of 13%. 

The three main meadow species, Meadow Brown, Gatekeeper and Ringlet appear to have done well and the Silver Y moth has jumped up the ranking.  These increases could be an effect of this year’s samples including an increased proportion of grassland sites.

More than half of our counts were carried out in countryside and a small number of sites yielded very large numbers of the meadow species which skews our results somewhat.  Our valley has a high proportion of grassland and no very large urban centre and so is not directly comparable with the national picture, but we can get useful information from our local data. 

The really alarming figure is the year on year switch in the proportion of the species seen.  Even with the large drop in Large Whites, this year’s top five accounted for over 81% of the total.  Last year, the same species accounted for only 53%.  Put the other way, the 15 least seen species dropped from 47% of the 2,090 butterflies recorded locally in 2023 to just 19% of the 2,250 seen this year. 

Sixteen of the 20 target species saw a per count drop from 2023 to 2024, 13 of them dropped by more than a half although some of these, such as the Marbled White and Jersey Tiger, occur in such small numbers in Devon that a small change in numbers seen will have a dramatic impact on the per count measure.

Numbers of the Large and Small Whites have more than halved locally.  The two species are often the most easily noticed butterflies in towns.  They are commonly lumped together as the ‘Cabbage White’ because they are very similar and both like to lay their eggs on cabbage plants which the caterpillars will destroy almost totally.  Their combined score dropped from 2.28 per count in 2023 to 1.12 in 2024.  This fits with the national trend where numbers per count have declined over the last few years.

National Decline of ‘Cabbage Whites’

On the other hand, the local numbers of Peacocks and Red Admirals per count appear to have halved from last year but nationally they are staging a recovery.  The local recorded change may be another effect of the shift in the proportion of grassland sites surveyed from 2023 to 2024.

Some gardeners may be pleased by the apparent decline in the ‘Cabbage White’, but the loss is a sign of other problems and not a cause for celebration.  Butterflies are among the poster species for the conservation movement, their fortunes are easily visible for the public to notice.  Their decline is being mirrored across the invertebrate world and is a signal that we need to do more if we are to maintain the biodiversity that sustains us ultimately.

The once common Small Tortoiseshell has almost disappeared from our valley with just 5 recorded this year, the number per count has dropped from last year’s already low figure.  In 2023 they were recorded in just one in ten counts but this year that has dropped to one in thirty five counts.  They are 17th equal with the Jersey Tiger, which has never been common in Devon, with just 5 butterflies seen in the 178 counts, a count rate of 0.03, down from last year’s 0.11 butterflies per count. 

In the previous 10 years of the Big Butterfly Count the national Small Tortoiseshell sightings have dropped by nearly one third, this year’s results show a further loss.  Ten years ago the Small Tortoiseshell was the sixth most common species in the national Big Butterfly Count but they are now down to 14th.  In 2023, more than 45,000 were recorded at a rate of 0.36 per count, this year only 12,700 were recorded at a rate of 0.10 per count.  The annual figures do fluctuate but the trend is a disastrous long term decline in the sightings of this beautiful butterfly that was so common in my childhood. 

The Decline of Small Tortoiseshell Sightings

However, a report from Malcolm Hull, chair of the Hertfordshire and Middlesex Branch of Butterfly Conservation suggests this may be a seasonal effect on the count rather than a wholesale decline.

https://butterfly-conservation.org/news-and-blog/where-have-all-the-small-tortoiseshells-gone

Small Tortoiseshells emerge in the spring, mate and breed.  They then feed up in preparation for hibernation which can begin in late June and early July, the butterflies seeking out refuges in places such as sheds and outbuildings.

Malcolm has recorded that more and more of the butterflies in his area are entering hibernation in June and so are not flying when the count begins in late July.  More work needs to be done to investigate how much of the recorded per count change is a real drop in population and how much is a shift in flying months.

The Brimstone is another case where seasonality may give a misleading picture.  By modern standards, Brimstones are reasonably common in the valley and are seen frequently, but only 2 were recorded in the 178 local counts, joint bottom of our local league table with the Green-veined White and a mean count frequency of 0.01.  Nationally, 8,793 were recorded with a mean count frequency of 0.06.  This is possibly a reflection on our mild climate.  Brimstones have a single generation per year and we see them as early as January and February and it is likely that their flying season is over by late July.  In the national data, Brimstones have consistently scored much more than our local 0.01, the mean score over the last six years is 0.73.  I would expect the distribution of sightings to focused further north but the Big Butterfly Count online map cannot be set up to show this.

Other species have different regional distributions which might explain discrepancies between the local and national count scores.  The Silver Y is a day flying moth with a population that is boosted by migration from France in some summers.  The local per count score was much higher than the national score, 0.53 per count and 0.03 respectively.  This is likely to include two factors, south cost counts have a higher mean score than further north, and a single outlier count that provided 27% of the local total.

A Slow Start

When this year’s Big Butterfly Count began there were frequent comments across social media that people were seeing very few butterflies and that the same was true for other insects such as bees.  Speculation centred on the weather, particularly the exceptionally wet spring, delaying the emergence of species that overwinter underground or at the base of rank grasses in unseasonally wet grassland.  Another possibility is that many of the over-wintering pupae and larvae will have drowned.  It is known that spring weather will impact on summer butterfly populations but only national data examining any correlation between the spring rainfall and butterfly count data in different regions could endorse or refute these ideas.  Hopes were expressed that things would improve as the weather improved. 

The slow start did improve.  In the first week of the count the mean number of butterflies per 15 minute count locally was 6.36.  This jumped to 15.20 in week two and was maintained at 15.62 in the third week, but was this a weather effect?

The weekly count pattern was similar but less marked in the national figures with week one 6.34 butterflies per count, week two 7.41,and  week three 7.53.  The discrepancy between the local and national pattern can be explained by the relatively small number of local counts, 55 in week one rising to 75 in week three.  This will have allowed a small number of outliers to have a disproportionate effect on the figures.  There are four individual counts with numbers of Meadow Browns or Gatekeepers so high one has to wonder how the recorder counted them in15 minutes without some double counting.  If you remove these from the data the week two mean per count drops from 15.20 to 12.64 and week three drops from 15.62 to 12.81.  The much larger number of counts in the national data, 39,000 in week one rising to nearly 57,000 in week three, will have ironed out any impact from outliers.

Weather

It is known that butterflies tend to be more active on warm sunny days and the weather across the three weeks did improve.  Week one had five consecutive days with some rain in Sidmouth and this came after the five previous days of heavy rain.  Then there was a break and most of week two was dry although there was one day, 25th July, when Sidmouth had 12mm of rain.  Week three was dry.  The first two weeks were not cold, daily maxima averaging 21o while the week three mean warmed to 25o with the 30th and 31st July reaching 29o and 30o. 

Sidmouth Rainfall Record

Any interplay between rainfall and temperature will be complex and it must be remembered that the small number of data available means any apparent effects will be interesting but not statistically reliable.

Whether or not the spring rain affected overall numbers, rain on the day did appear to affect the counts in week one.  The mean count score on wet days was 4.42 but this rose to 8.88 on the dry days, but that is still only just over half of the mean for weeks two and three.  There were only two counts on the two wet days in week two and those counts recorded zero and a single butterfly respectively. 

Counts in gardens tended to have much lower scores than those out in the countryside and there was a possibility that the wet weather restricted volunteers to garden counts, but analysis shows this was not the case.  In week one which had a mix of wet and dry days there were fewer counts on the wet days, 31 over five days compared to 24 on the two dry days, but there was a higher proportion of garden counts on the dry days in week one compared with the days when it rained. 

As expected, temperature appears to have had an effect on count scores with warmer days averaging higher counts, the tipping point appeared to be at 21o.  There is the complication that the cooler days tended also to be wet days.

The outlook is troubling, climate change is predicted to give us warmer summers which will see butterflies flying more often, but we are also likely to have wetter springs which will impact on breeding numbers.

Habitats

Unsurprisingly, the location habitat has a significant impact on the number of butterflies seen, but also on the balance between species in the count.  Local counts were divided into five habitat categories, gardens, fields (agricultural land or large public spaces being managed as meadows), parks (large public spaces with mown grass), woodland (including woodland glades and rides), and heath.  The results for the five categories are summarised in Table 3.

The woodland sites had the highest counts with a mean of 24.71 per count, but with only 7 results this is not a reliable figure to base any judgements. 

There were only 14 park sites but with the difference in count scores being so wide it is valid to say that our large open spaces with mown grass are home to fewer butterflies than are our fields, areas of long grass and mixed vegetation, 4.93 against 21.29 per count.  One site epitomised the difference, the cemetery.  Most of the cemetery is currently managed with short grass, but the southern section has been left uncut, it is now an area of rank grass with invading bramble.  A count in the eastern half of the short grass area yielded a single Meadow Brown.  On the same day, a count in the uncut area yielded 24 butterflies of 4 species, mainly Ringlets.  To reinforce the point, the eastern half which is area now managed as cut grass was left uncut in previous years and last year the area yielded 42 butterflies of 8 different species.

The majority of the local counts were conducted in fields and gardens, 65 and 63 counts respectively.  Members of each category will be very mixed but the difference between their results is so marked that it is safe and unsurprising to say that more butterflies are found in fields than in gardens, 21.29 against 4.65 per count.  Thinking of the demographic of the volunteers, it is likely that their gardens have a wildlife friendly element and the mean count score would be even lower if a random selection of gardens was made.  A single nature friendly garden has limited chance of success if it is isolated, nature needs connectivity.

          Delderfield Meadow Mean Count 14.7                                 Byes Lawns Mean Count 4.0

There were 30 local counts of zero or only one butterfly.  Sixteen of them were in week one, 8 in week two and only 5 in week three and weather may have been a factor.  The majority, eighteen, were in gardens and only three were in a meadow or pasture.

The top thirty counts recorded 20 or more butterflies.  Of these, only 3 were in week one, 11 were in week two and 16 were in week three.  Of the top thirty, only one was in a garden and that garden is on the edge of the urban area backing onto fields.

The different site categories had different diversities, the numbers of species seen per count and across the category, and the proportions of those species.  Looking at the species seen within each category (Table 3), there are a number of conclusions suggested by our small data sample. 

Woodlands accommodate a wide range of species, but the style of woodland is important here.  Our 7 counts were sited in deciduous woodland with open glades and rides.  You see very few butterflies in dense woodland and will probably see none in conifer plantations.

It is clear that you are not only likely to see more butterflies in fields and than parks, but also a wider range of species during individual counts, a mean of 4.02 compared to 2.5, and across the category with 17 of the 20 target species seen in the fields while nearly half the target species were missing in the short grass areas.

Despite the name, Meadow Browns are generalist butterflies that have found a home across a wide range of habitats.  A key factor is likely to be the food plants of the caterpillars which are happy to eat a range of finer grasses.  Gatekeeper caterpillars have a similar diet. 

Large White and Small White caterpillars feed on Brassicas such as Cabbage and Brussels Sprouts, but the Brassica family is large and widespread which is why these ‘Cabbage Whites’ are common across the UK not just in vegetable patches.

The Ringlet is more specialised than the Meadow Brown and Gatekeeper.  Ringlet caterpillars prefer coarser tussock forming grasses which are not available in parks and gardens generally.  The highest local counts of Ringlets were mainly in meadow areas and our heathland which has large areas of tussock forming grasses.  The Ringlet was the 3rd most common butterfly in local counts with 0.69 per count but only 7th nationally with just 0.34 per count, a measure of the dominance of agricultural grassland in our valley. 

Habits

Looking beyond the crude number per count data, you can have two species with similar scores but different patterns of sighting.  In the national data, Six-spot Burnets and Green-veined Whites had the same mean per count rate of 0.14 because they had very similar gross counts, 18,370 and 19,297 respectively.  The difference is that Burnets tend to be gregarious and their total number were seen in only 3,500 counts while the more solitary Green-veined Whites were recorded in nearly three times the number of counts, 9,900.

When they were seen, the mean score for Burnets was 5.20.  Meadow Browns also tended to be seen in groups and have a mean score of 4.32 when they were seen, but they were seen twelve times as often and nearly 200,000 were recorded in total compared to nearly 20,000 Burnets.

Conclusion

The Sid Valley is one of the best locations for butterfly populations and diversity because of its climate and habitat diversity but even this favoured area shows troubling signs of decline and this is a clear indicator of an overall problem facing our total biodiversity.  There is already a lot of work going on to mitigate the effects of changing climate and habitat degradation.  Hopefully, there are enough people doing many small things that will add up to a landscape scale reversal of the decline.

Appendix 1  Big Butterfly Count 2024 National Results

Appendix 2 Big Butterfly Count 2024 Sid Valley

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