Hogstad, O. “Sexual Bill Dimorphism Supports Separation of the Woodpecker Genera Dendrocopos Koch (1816) in the Palaearctic and Picoides Lacépède (1799) in the Nearctic”. Ornis Norvegica, Vol. 31, Jan. 2008, pp. 183-8, doi:10.15845/on.v31i0.167.