Swann, H. Kirke, 1871-1926. A Synopsis of the Accipitres (Diurnal Birds of Prey), Comprising Species and Subspecies Described up to 1920, with their Characters and Distribution. 2nd edition, revised and corrected throughout. London: Privately Printed for the Author, 1921-1922.
This post originally published on the Smithsonian Collections blog, April 16, 2013.