Interbreeding Members of the Sphyrapicus varius Group (Aves: Picidae) in Oregon
Evidence from specimens collected in south-central Oregon provides further support for considering Sphyrapicus nuchalis and S. ruber as distinct species of the S. varius group.
Browning, M. Ralph
"Interbreeding Members of the Sphyrapicus varius Group (Aves: Picidae) in Oregon,"
Bulletin of the Southern California Academy of Sciences:
Available at: http://scholar.oxy.edu/scas/vol76/iss1/7