This is from Wikipedia.
“John James Audubon was a French-American ornithologist, naturalist, hunter, and painter. He painted, cataloged, and described the birds of North America in a manner far superior to what had gone before…Audubon developed his own methods for drawing birds. First, he killed them using fine shot. He then used wires to prop them into a natural position, unlike the common method of many ornithologists, who prepared and stuffed the specimens into a rigid pose. When working on a major specimen like an eagle, he would spend up to four 15-hour days, preparing, studying, and drawing it. His paintings of birds are set true-to-life in their natural habitat. He often portrayed them as if caught in motion, especially feeding or hunting. This was in stark contrast to the stiff representations of birds by his contemporaries.
He worked primarily with watercolor early on. He added colored chalk or pastel to add softness to feathers, especially those of owls and herons. He employed multiple layers of water coloring, and sometimes used gouache. All species were drawn life size which accounts for the contorted poses of the larger birds as Audubon strove to fit them within the page size. Smaller species were usually placed on branches with berries, fruit, and flowers. He used several birds in a drawing to present all views of anatomy and wings. Larger birds were often placed in their ground habitat or perching on stumps. At times, as with woodpeckers, he combined several species on one page to offer contrasting features. He frequently depicted the birds’ nests and eggs, and occasionally natural predators, such as snakes. He usually illustrated male and female variations, and sometimes juveniles. In later drawings, Audubon used assistants to render the habitat for him. Going beyond faithful renderings of anatomy, Audubon employed carefully constructed composition, drama, and slightly exaggerated poses to achieve artistic as well as scientific effects.
Audubon’s influence on ornithology and natural history was far reaching. Nearly all later ornithological works were inspired by his artistry and high standards. Charles Darwin quoted Audubon three times in On the Origin of Species and also in later works. Audubon’s field notes comprised a significant contribution to the understanding of bird anatomy and behavior. ‘Birds of America’ is still considered one of the greatest examples of book art. Among his accomplishments, Audubon discovered twenty-five new species and twelve new subspecies.”