Flash mobs spread to Europe: "...about 40 people in the middle of a busy street took out their mobile phones and shouted, 'yes, yes!' and then applauded," according to The New York Times.
If all of the sudden scores of people converge upon your roof chirping and squawking like birds, it may just be a "Flash Mob" or as the originator of the fad calls them "inexplicable mobs". The inexplicable nature and lack of apparent agenda seems to widen the appeal of flash mobs says this CNN article.
Take a read :-) - it's a cheerful little article.