Spy pigeons fitted with metal rings and "invisible" strings have been captured. http://www.telegraph.co.uk/news/Iran-arrests-pigeons-spying-on-nuclear-site.html Which reminds me: The baby pigeon complained to his mother before flying a long distance, 'I can't make it, I'll get too tired.' His mother replied, 'Don't worry, I'll tie a piece of string to one of your legs and the other end to mine.' The baby started to cry. 'What's wrong?' asked the mother sympathetically. 'I don't want to end up being pigeon towed.'