Postage stamps aren't just for sending love letters; they're love letters in themselves—unique, commemorative works of art that embody history, nature, and national identities. So what happens when you add birds to the mix? Well, as evidenced by World Stamp Show, which comes to New York City this weekend, you end up with an array of beautiful, thumb-sized portraits of tanagers, eagles, owls and waterfowl, worth anywhere from 20 cents to $20 million. The culture of making and caching stamps dates back to the 1840s, when British postal administrator Rowland Hill inspired a new artistic medium and obsession with the help of a single adhesive label: the “Penny Black” stamp, portraying Queen Victoria. With the birth of the first stamp came the rise of the first philatelists, otherwise known as stamp collectors. While some philatelists strive for quantity, others are more niche in their cravings. Thematic collecting has gained popularity over the last 30...