SCOTLAND’S butterfly populations have increased over the last 40 years probably due to warmer summers, according to a new report,

The latest Scottish Biodiversity Indicator published by NatureScot examines the long-term trend for butterflies from 1979-2019.

While the trend shows a moderate increase overall, the picture is complex, with species faring differently.

Some butterfly populations in the UK continue to shift northwards as a response to climate change.

Those expanding their range northwards into southern Scotland include small skipper and most recently the white-letter hairstreak.

Monitors were also surprised to find a small colony of Essex skipper in the Scottish Borders - one hundred miles further north than the nearest colony south of the Border - although it is thought possible that caterpillars ere transported north on hay.

Ringlet, peacock, and orange-tip butterflies show significant long-term

population increases, while the small heath is also on the up.

Meanwhile speckled woods have expanded their range from their strongholds n Highland and south-east Scotland into new areas.

Regular migrant butterflies such as the red admiral are also increasing over the long-term as a response to warming.

Species in long-term decline include the small tortoiseshell, which may be due to poorer overwinter survival in warmer and wetter winters.

Grayling have also declined but the small pearl-bordered fritillary and pearl-bordered fritillary have increased significantly. These two species may be benefitting from native woodland planting and targeted management at specific sites.

Simon Foster, NatureScot Trends and Indicators Analyst, said: “We know that habitat loss, climate change and urban development are among the key factors that are affecting butterfly populations.

“We’re working with partners across Scotland on a range of projects to help our butterflies and other pollinators thrive, from habitat creation and management to promoting wildlife friendly gardening and best practice guidance for developers.

“Butterflies can also benefit greatly from more people getting involved in citizen science. If you would like to help why not join the Butterfly Monitoring Scheme and get involved with surveys? It’s easy, fun and can help us improve our knowledge of what is happening where, giving us the best chance of targeting conservation measures most effectively.”

The Scottish Biodiversity Indicator is a multi-species index compiled by Butterfly Conservation (BC) and the Centre for Ecology & Hydrology, using data primarily from the UK Butterfly Monitoring Scheme (UKBMS).

The results contrast with the recently published Big Butterfly Count, which reported a record 70% decline in butterflies this year - the worst annual crash in the colourful insects north of the Border since its launch in 2010.

NatureScot said the annual snapshot survey can be influenced greatly by weather and timing of peak flight periods, however. The trends assessed in the indicator highlight changes over a longer time period, smoothing out the large annual fluctuations.

Dr Tom Prescott, BC’s Senior Conservation Officer in Scotland, said: “The Scottish Biodiversity Indicator offers a robust dataset and it is encouraging that species are doing well but there will always be winners and losers from climate change.

“These figures are largely positive in the respect that species are doing well and spreading to new areas, but there are concerns that with our damper milder winters species like the small tortoiseshell appear to be suffering.

“Some species we have started to see arriving and moving in Scotland have been spreading north through England and banging on the Border for some time.

“There was a small colony of Essex skipper which seemed to appear from nowhere a good hundred miles from the nest nearest colony. One thought is that it may have come over the Border to Dumfries and Galloway as caterpillars or eggs on hay.”