This video says about itself:
Scarce Tortoiseshell Feeds on Oak Sap ヒオドシチョウがミズナラ樹液を吸汁
9 February 2014
A Scarce Tortoiseshell (aka Yellow-legged Tortoiseshell; Nymphalis xanthomelas japonica, family Nymphalidae) feeding on the fermenting sap of an oak tree (Quercus crispula, family Fagaceae). October 2013 in Japan.
Translated from the Dutch butterfly foundation:
Monday, March 9th, 2015
It was a beautiful sunny weekend and that was evident from the butterflies flocking, having left their wintering areas. Many people on Saturday and especially on Sunday saw their first butterfly of 2015. The brimstone was absolutely the most frequent species, but most special were the six scarce tortoiseshells.
Nearly 1500 brimstone butterflies were seen on 7-8 March.
Scarce tortoiseshells are an east European and Asian species. Last year, they were seen in the Netherlands for the first time. It turns out now that some have survived the winter.