This past May, when it finally sank in that I was going to be stuck at home for a very long time because of the pandemic, I took up a hobby that had never especially appealed to me before: birding. I cleaned my neglected bird feeder and filled it with seed, retrieved my binoculars from a gear bag in the basement, and started having my morning coffee outside, slowly learning to identify species based on body size, feather colors, beak shape and song. At last count I had logged 39 species from the confines of my suburban backyard. These hours spent observing birds—the goldfinches congregating at the feeder, the pileated woodpeckers drumming in the trees, the turkeys strutting across the lawn, the ruby-throated hummingbirds hovering above their favorite blooms, the red-shouldered hawks circling overhead—have given me a newfound appreciation for their diversity. And I am seeing only a sliver of the actual richness of avian forms. With more than 10,000 species alive today, birds constitute the most diverse group of land vertebrates (backboned animals) on Earth. How did they come to be so spectacularly varied?