Butterfly Transect Data Summary 2021


The 2021 season was our 15th year of monitoring on the two sites.  It got off to a very slow start with poor weather during most of April.  Conditions improved in late spring and summer, when it was generally warm but often cloudy with higher than average rainfall.  On Fleam Dyke, transect walks were completed for 25 of the 26 weeks but one of these was considered invalid because weather conditions did not reach the minimum requirements.  Also because of poor weather conditions, a transect walk was not done in Week 8.  On the Roman Road, counts were also completed for 25 weeks and results from all of these were accepted.

A total of 25 species was recorded on each site. The only differences in the range of species were a Dark Green Fritillary on Fleam Dyke and a single White-letter Hairstreak on the Roman Road, the first recording for this species in a transect count on this site.  Total butterfly sightings on the Roman Road were very similar to the previous three years.  On Fleam Dyke, it was the highest total so far, mainly due to an increase in the number of Chalkhill Blues.

Looking at the data for individual species, it was again a season of mixed results.

As indicated above, the first of our three habitat specialist species, the Chalkhill Blue, had an excellent year on Fleam Dyke, The index* of 2657 was 46% higher than in 2020 and 20% above our previous maximum of 2215, achieved in the year 2015.  On the Roman Road, the very small population of this species continued to hang on in 2019 and 2020, with indices in single figures but this year the index increased to 43.

The second habitat specialist species, the Green Hairstreak, had a good season on Fleam Dyke with an index of 26, the highest since 2011. The population is now much less concentrated in area than it was when we started monitoring.  On the Roman Road, two were recorded, following a single sighting in 2020.

After a good year in 2020, only one Dark Green Fritillary was recorded in transect counts on Fleam Dyke and none on the Roman Road.

Moving on to the wider countryside species, Small/Essex Skippers showed a further reduction in numbers on Fleam Dyke but had a relatively good year on the Roman Road with numbers similar to those recorded in 2020.  Although it is not possible to determine the species for every individual sighted, this year only Essex Skippers were actually identified.  The Large Skipper showed an improvement compared with 2020 on both sites with indices around the 15-year average.

The Brimstone had a good year on both sites and on the Roman Road, where it did particularly well in the latter part of the season, we recorded our highest index to date. Both the Large White and the Green-veined White had a poor season with relatively low numbers on both sites. However, Small White numbers were similar to those recorded in 2020 and it was still the most abundant butterfly on the Roman Road. Orange-tip numbers were around average on the Roman Road but probably, because a critical week was missed on Fleam Dyke, an index could not be calculated.  The numbers recorded in other relevant weeks were very low.

The Common Blue had another very poor season and Brown Argus numbers, although around average were lower than in 2020.  Holly Blue numbers were slightly down on both sites.  Sightings of the Small Copper are quite rare in our transect counts but this year, four were recorded on the Roman Road and one on Fleam Dyke.

It was a mixed year for the vanessids.  The Red Admiral had a good season, with the highest number to date on Fleam Dyke and the best year since 2017 on the Roman Road.  The Small Tortoiseshell was down in numbers on both sites and had a particularly poor season on the Roman Road.  Peacock numbers continued to increase, with the best season since 2014 on Fleam Dyke and the best since 2015 on the Roman Road. The indices for the Comma remained in single figures on both sites.  With indices of 118 and 108 on Fleam Dyke and the Roman Road respectively, 2021 was a good year for the Painted Lady, a migrant species, with numbers similar to those recorded in 2019 but still much lower than those achieved in 2009, the previous good year.

The Marbled White was again one of the highlights of the year with an index of 101 on Fleam Dyke, the highest to date and 47 on the Roman Road, lower than in 2019 and 2020 but still much higher than in previous years.  Hopefully, this is further confirmation that this species is becoming established on our two sites, after many years of zero or very low numbers in the transect counts. The number of Gatekeepers on Fleam Dyke showed a further reduction but on the Roman Road it was the best year since 2009. The other satyrids fared less well. The Speckled Wood had a generally poor season but numbers on the Roman Road showed some improvement relative to 2020, which had produced the lowest counts in the 14 years of monitoring.  The Meadow Brown had another poor year, particularly on the Roman Road, where there was a 61% reduction compared with 2020 and it was the worst year on record. The Ringlet also had another very poor season on both sites with the lowest numbers so far recorded.  After a very good year in 2020, Small Heath indices were around the average but continue to be much lower on the Roman Road than on Fleam Dyke.

* The index is a measure of abundance, which is calculated for each species.  It is the total number of butterflies recorded in the transect counts, adjusted, where necessary, using estimated numbers for any missing counts or where weather conditions were not within the guidelines.

Chalkhill blues on carline thistle, Fleam Dyke