FLEAM DYKE AND ROMAN ROAD

The 2023 season was our 17^th year of monitoring on the two sites.  Following the hot, dry summer of 2022, this year was more variable with occasional hot spells but also long periods of cloudy conditions with rain.  On Fleam Dyke, transect walks were completed in all weeks of the season and on the Roman Road, counts were completed in 24 of the 26 weeks.

A total of 25 species was recorded on each site. Twenty three species were recorded on both sites but in addition, there were Green Hairstreaks and our first record of Adonis Blues on Fleam Dyke, and on the Roman Road we recorded a single Small Blue and 2 Small Coppers. Total butterfly sightings on the Roman Road increased by 44% compared with the poor 2022 season and on Fleam Dyke, there was a 14% increase to give the highest number we have ever recorded. Species that did particularly well in comparison with previous years were the Holly Blue, Brimstone and Red Admiral on both sites and the Brown Argus, Marbled White and Gatekeeper on Fleam Dyke.  Some other species had a particularly poor season, notably the Small Tortoiseshell and the Ringlet on both sites.

Looking at the data for individual species in more detail, the first of our habitat-specialist species, the Chalkhill Blue, had another good year on Fleam Dyke but the index* was down 5% compared with 2022 and 16% compared with our record year of 2021, when the index was 2657.  On the Roman Road, the very small population of this species is still surviving but the index fell from 19 in 2022 to 14 this year.  We can now add two more blues to our list of habitat-specialist species. This year, on Fleam Dyke, we recorded the Adonis Blue for the first time, with 5 seen on Fleam Dyke in early June.  On the Roman Road,another Small Blue was recorded, following a single sighting in 2022.

Another habitat-specialist, the Green Hairstreak, had an average year but the index was more than 60% lower than in our record year of 2022.  On the Roman Road, none were recorded in 2023.  Similarly, no Dark Green Fritillaries were recorded this year.

Moving on to the wider countryside species, Small/Essex Skippers had another poor year, particularly on Fleam Dyke. The Large Skipper alsoshowed reduced numbers to well below average figures on both sites.

The Brimstone had an excellent year, with the highest number ever recorded on Fleam Dyke and numbers near our previous maximum on the Roman Road. The Large White and Green-veined White increased in numbers on both sites with particularly significant increases on Fleam Dyke.  The Small White showed slight increases relative to 2022 and was still the most abundant butterfly on the Roman Road. The Orange-tip index increased to around average on Fleam Dyke but was slightly above average on the Roman Road.  No Clouded Yellows were recorded this year.

The Brown Argus had an excellent season with the highest number we have ever recorded on Fleam Dyke and an index approaching the best year on the Roman Road.  Of all the resident species present on our sites, the Common Blue probably shows the greatest variation in numbers from year to year.  It had a very poor season in 2023 on both sites but in other previous seasons, numbers have been even lower.  Holly Blue numbers were the highest we have recorded with increases of 136% and 378% respectively on Fleam Dyke and the Roman Road.  Two Small Coppers were recorded on the Roman Road this year.

It was another mixed year for the vanessids.  Following a big reduction in numbers in 2022, the Red Admiral showed a large increase in 2023, giving indices similar to those of 2021.  The Small Tortoiseshell had a very poor season with the lowest numbers recorded for more than 10 years.  Conversely, Peacock numbers increased and on the Roman Road it was the best year since 2015.  The indices for the Comma showed some increase but total numbers of this species remain low.  Numbers of the Painted Lady, a migrant species, were again very low after two moderately good years in 2019 and 2021.

The Marbled White had another excellent year. On Fleam Dyke the index increased by 40% and again, the population was concentrated in the area southeast of the A11 towards Bedford Gap.  This species also continued to do well on the Roman Road but because a critical week was missing from the counts, an index could not be calculated. However, even without that critical week, the total number recorded was similar to 2021 and 2022. The Marbled White can be regarded as one of our success stories as it was recorded only in single figures on both sites up to 2017. The number of Gatekeepers on Fleam Dyke increased by nearly 150% compared with 2022 to give our highest total to date.  Numbers on the Roman Road were much lower but there was some improvement on the record low number recorded in 2022. The Speckled Wood showed a healthy increase in numbers relative to 2022 but did not reach the levels achieved in some previous seasons, particularly 2014, which was the best year to date. The Meadow Brown showed some improvement compared with 2022 but indices on both sites were still below average. The gradual decline in Ringlet numbers, apparent now over a number of seasons, has continued.in 2023 with indices of only 1 and 4 respectively on Fleam Dyke and the Roman Road. In our best season to date, which was 2014, both indices were well over 300.  The Small Heath increased in number on both sites but the indices were still below the maximum levels achieved in 2020. Numbers have always been 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.    

.