Data from the UK Butterfly Monitoring Scheme (UKBMS) for the Marbled White butterfly (Melanargia galathea), a fairly widespread species that flies in a single generation per year in the UK.
Most survey sites are chosen opportunistically and represent fixed transects of variable and often unknown length, which are walked between two and 26 times per year starting in early April. Butterflies are counted within an imaginary 5m x 5m 'moving box' along the transects (i.e., according to a Pollard walk protocol).
We restrict the data to sites where the marbled white was ever recorded, the years 1991–2015 and those 80 sites where counts took place in at least 20 of these 25 years.