This is a scarce tortoiseshell video from Finland.
Soon, people did not see these rare, usually east European, butterflies any more. Did they fly on to England, or to Belgium? A few scarce tortoiseshells were seen there indeed; but how about all the others?
On 2 August 2014, warden Luc Knijnsberg was in Dutch nature reserve Bergen Zuid. Then, a thunderstorm started. Luc entered an old World War II bunker, now used by bats for wintering. In that bunker, he found thirteen wintering peacock butterflies (yes, August is a summer month, but for some butterflies, winter then starts already). Luc found not only peacocks, a common species, but to his surprise also two scarce tortoiseshells. So, this solved the riddle why so many of them arrived in the Netherlands; and so few in Belgium or England.
So, people counting wintering bats should look out for scarce tortoiseshells as well.