Here is a photo of the imprint left by a bird flying into a window. Its from a flickr group called Bird Imprints on Glass which features 122 photos.
I think its really interesting the way various flickr users around the world make observations of simple phenomena like this independently and that they are then compiled through the site. This group and the previously posted chewing gum trees group remind me of some of the work I did as a student. I'm glad I don't work like that now because I don't think i could compete with the combined resources of the Flickr community.