I've not seen butterflies do that. However, we have large moths called bogongs which swarm like that. I have been in buildings where they swarm up against the windows and block out the light. They have a very fat, edible body and are generally as long as one's middle finger, but can be larger. I will try to attach a photo of the first one I have spotted this year.[ATTACH=CONFIG]332707[/ATTACH][ATTACH=CONFIG]332708[/ATTACH]