The Banking on Butterflies project aims to improve our understanding of how to support butterflies in the face of climate change using mounds of earth called ‘butterfly banks’. The banks we have built are E-shaped, with dimensions of 15m by 12m by2 m, and are designed to provide a range of temperatures, shelter and shade to help butterflies control their body temperature. The patches of bare ground on the banks, which remain even after several years of plant growth, are very important because the temperature there is often different to the temperature on nearby plants. The patches of bare, chalky soil also give an opportunity for the seeds of chalk grassland specialist plants to germinate without being shaded out by surrounding grass. These plants, in turn, are food for the caterpillars of chalk grassland specialist butterflies such as the nationally scarce Small Blue. 

The project started in Bedfordshire, with four banks at Pegsdon Hills Nature Reserve and four at Totternhoe Nature Reserve completed in 2021. In 2024, we built eight more banks in Wiltshire – four at the Wiltshire Wildlife Trust site Coombe Bissett Down and four at the RSPB’s Winterbourne Downs. In 2025, another four banks were built, this time at Trumpington Meadows in Cambridge. There are more exciting plans for additional banks in 2026 – hopefully expanding to another county! 

Baseline surveys of butterflies, ground-dwelling invertebrates, plant communities, and air temperature on the banks were conducted before and after they were built, as described in Matt Hayes’ previous blog – Banking on Butterflies - one year on. Every year thereafter, the team has tirelessly conducted butterfly surveys monthly throughout the summer at all butterfly bank sites and monitored ground-dwelling invertebrates using pitfall traps every autumn. With this longer-term data, we can draw conclusions not only about the immediate effect of the banks, but also about how they change over time as plants grow on them and they begin to mature and weather.

Rosalind Mackey – How butterfly banks affect ground-dwelling invertebrates

I used the ground-dwelling invertebrate samples to investigate how the banks have affected the diversity of ground-dwelling invertebrates over time. While the initial creation of banks caused a decrease in invertebrate abundance within the bank area, after a few years the banks held many more invertebrates than the surrounding grassland. They also held a slightly different assortment of invertebrates, suggesting that this added habitat diversity increased the diversity of invertebrates in the reserves. 

Because we used 22 pitfall traps per bank, I was able to look at the distribution of invertebrates across the bank area in great detail. Combining this with weather records from the time of trapping, I found that invertebrates use the northern and eastern faces of the banks to shelter from westerly winds. I didn’t find any changes in invertebrate distribution with temperature, but this was expected since the average temperature across my sampling times only varied by just over 3 °C.

Rosa Pollard Smith – How butterfly behaviour varies with temperature

As well as conducting the monthly butterfly surveys on the banks, organising the building of the Wiltshire banks, and liaising with the wider team about outreach and policy outcomes of the project, I conducted fieldwork for my masters project at the bank sites in Pegsdon, Totternhoe, Coombe Bissett, Winterbourne Downs and Trumpington. My masters focussed on understanding how butterfly behaviour (e.g. flying, feeding, resting, feeding, egg-laying) changed with temperature to aid predictions about how butterflies are likely to cope as temperatures rise and extreme heat events become more common. One thing I investigated was butterfly perch selection and how this changed with temperature, with the aim of informing management of the banks and wider reserves.

I tracked butterflies for up to 3 minutes, recording characteristics of everything that they landed on. I marked the perches that butterflies landed on with stickers, returning to them after the tracking period to take the temperature of the perch and air above it, record whether the perch was near shelter or in shade, and measure the height of the perch. 

I found that butterflies show stronger preferences for cooler, more sheltered, more shaded and higher perches as air temperatures rises. This provides further evidence that butterfly banks are really important for butterflies at high temperatures! My work suggests it is crucial to provide butterflies with these cooler areas along with shelter and shade at warm temperatures –  banks offer an effective means to do this.

Rosalind Mackey – What reserve visitors think of the banks

This spring and summer, I interviewed 213 reserve visitors across the Bedfordshire and Wiltshire reserves, asking about their thoughts on the banks. I found that people were generally positive towards the banks; around 80% of people liked the banks, with a similar percentage thinking they would be effective at mitigating the effects of climate change and conserving biodiversity. For many people, the belief that the banks would help conserve wildlife was enough to balance out the fact that people often found them ugly when they were new. Some people also commented on the fact that the banks became much more aesthetically pleasing over time, so even that negative perception may be short-lived. 

In addition to asking people’s opinions, I investigated how much of the information on nature reserve signs the average visitor remembered. To my surprise, people who had read the signs knew no more about the contents than people who had not read them, while the banks seemed to distract attention away from signs, rather than drawing readers to them. To me, this indicates that signs in nature reserves serve a different purpose than just education, such as increasing engagement or provoking discussion. The survey certainly provoked discussion – I had some wonderful conversations with people from a wide variety of backgrounds and viewpoints. 

Future directions

Our two master’s projects are far from the end of the work on these banks. Data collection continues, with the aim of collecting long-term data about the impacts of the banks over several decades. As we build more banks, we are achieving a better coverage of the diversity of chalk grassland habitats, meaning our results are relevant to an ever-increasing area of the country. In the meantime, the two of us are working hard on publishing our work in scientific journals, so that our discoveries can be used to inform future conservation work and research.