A tower made of bricks made of paper for pigeons where they can defecate and the excrements will then be collected and turned into fertilizer, by Arne Hendriks
"Pigeon poop used to be a highly valued fertilizer for food crops. The Pigeon Towers of Isfahan in Iran were industry-sized shit factories, housing thousands of pigeons and located among the agricultural fields of the Isfahan Valley. As the world is facing depleting stocks of readily available artificial fertilizers perhaps it is time to re-evaluate the city pigeon's bad reputation and appreciate them for their ability to create high quality shit. The tower is made from digested old newspapers."