There were many butterfly wing fragments floating in the shallows and washed up on the shore that evening. I did not see any intact butterflies. If I may just hazard a guess, perhaps a spate of bad weather killed a group (flock?) of migrating butterflies. Scavengers took care of the bodies, leaving behind the unpalatable wings. I’m not sure if butterflies in the region do actually migrate, or whether they migrate in discrete single-species groups, but it was certainly strange to see so many wings from just a single species of butterfly concentrated within the area.