Birds in two European cities have unwittingly become part of an artwork. A pair of artists installed a cage-like device on a rooftop in Copenhagen, Denmark, and later in Venice, Italy. When pigeons fly inside, a conveyor belt carries them through the contraption. It sprays the birds with non-toxic dye that fades over time. The machine then releases the tropical-looking pigeons back into their urban habitats. Some people see the project as a fascinating public artwork. But critics say that trapping and dyeing the birds is a form of animal abuse.