Do they migrate? You can hear the English channel from our nursery and the next stop is France, so we often see the first migrant birds flying over. But yesterday we had about 3 or 4 brimstone butterflys flying over every hour, heading north.
I cannot remember noticing this before, It might have been the higher temperatures which were nudging 20c for the last few days coaxing them out of hibernation, usually it is the tortoiseshells that emerge first, but I have not seen any of those yet.