When I was doing Bartels Science Illustration Intern at Cornell Lab of Ornithology, the book "The Laws Guide to Drawing Birds" was always on my table where I worked. This book taught me the key points of drawing birds, such as setting the proportion of head vs body or eye vs beak, the feather orientations, and color controls for bird painting. I was so thrilled to meet the author, John Muir Laws at the GNSI meeting! Not only artistic skills, but also he has a talent in presentation/teaching. His talk was fantastic!
His website: http://www.johnmuirlaws.com/