John Cassin (September 6, 1813 – January 10, 1869) was an American ornithologist. He is considered to be one of the giants of American ornithology, and was America's first taxonomist, describing 198 birds not previously mentioned by Alexander Wilson and John James Audubon. Cassin drew, printed and hand-colored many of the illustrations for the Pacific Railroad Surveys as the western regions of the United States were explored. He was named curator of the Philadelphia Academy of Natural Sciences in 1842, and after John T. Bowen's death, he took control of the Academy. His "Illustrations" was intended to be a 3-volume work, but financial considerations prevented more than one volume being published.
Cassin was born near Philadelphia on September 6, 1813 to a Quaker family whose members had distinguished military and naval service. From his early youth, however, John's favorite study was ornithology. He contributed descriptions of new species of birds and synoptical reviews of various bird families to the Philadelphia Academy of Natural Science. His publications included "Birds of California", a handsome octavo volume, containing descriptions and colored engravings of fifty species not given by Audubon; a "Synopsis of the Birds of North America"; "Ornithology of the United States Exploring Expedition"; "Ornithology of the Japan Expedition"; "Ornithology of Gillis' Astronomical Expedition to Chili"; and the chapters on rapacious and wading birds in the "Ornithology of the Pacific Railroad Explorations and Surveys".
The best known of Cassin's many publications are his Illustrations of the Birds of California, Texas, Oregon, British and Russian America (1853-56), and Birds of North America (1860), co-authored with Spencer Fullerton Baird and George Newbold Lawrence.
He is memorialized through the names of several North American birds, all of which breed in the western U.S.: Cassin's Auklet, Cassin's Kingbird, Cassin's Vireo (formerly a subspecies of Solitary Vireo), Cassin's Sparrow and Cassin's Finch.