Writer Nathanael Johnson realized, after repeated questions from his daughter during their walks, that he knew very little about the flora and fauna in his neighborhood. He set out to discover the stories behind these everyday plants and animals in his book, "Unseen City: The Majesty of Pigeons, the Discreet Charm of Snails & Other Wonders of the Urban Wilderness." We'll talk to Johnson and an expert on urban wildlife about the species that thrive in Bay Area cities. And we'd like to hear from you: what animals and plants captivate you on your urban walks?