Character variation and evolution of sibling species in the Empidonax difficilis-flavescens complex (Aves Tyrannidae) - Johnson, Ned K. (Berkeley University of California Press, 1980)
Bibliography: pages 141-151.
Keywords: Pacific-slope flycatcher -- Variation; Pacific-slope flycatcher -- Evolution; Yellowish flycatcher -- Variation; Yellowish flycatcher -- Evolution; Birds -- Variation; Birds -- Evolution