Book excerpt: From the discovery of the fossil Archaeopteryx to more than 10,000 different documented species today, birds have become the second most diversified class of vertebrates on Earth. Birds have evolved extensively since they first emerged in prehistoric times--but that diversity could dwindle and even vanish unless we take steps to conserve their habitats, ensuring that they sustain their numbers and their variety. This natural history of birds starts in the distant past--going back to the Jurassic, Cretaceous and Paleogene periods--in order to get a broader understanding of the birds that we see today. Chapters cover their lives, breeding, flight, migration and more, while also highlighting some especially unique bird fossils, such as the Pelagornis Sandersi, which had a wingspan of more than 20 feet. Also included are chapters on the loss of needed habitats, the current decline of native birds, and what can be done to reverse it.