IOC World List: The Yellow-rumped Warbler species group (November 2011)
The Yellow-rumped Warbler (Dendroica coronata) comprises four distinct geographic populations currently treated as subspecies. Two of these, the migratory Myrtle Warbler and Audubon Warbler of North America were long considered different species until the AOU (1973) lumped them based on initial studies of hybridization in a narrow zone of contact in the Canadian Rockies.
Two new papers address these taxa, including recommendations for recognition of all four as biological species. We accept the recommendations as a step forward, even though the NACC (2010-A-4) ) considered this premature.
Milá et al (2007) analyse the genetic relationships and biogeographical history of all four populations. The nonmigratory Black-fronted Warbler (D. nigrifrons) and Goldman’s Warbler (D. goldmani) differ from the migratory taxa in plumage color pattern, substantial genetics distances, and of course annual cycle. They are morphologically and genetically distinct allopatric species. The extent of hybridization between the Black-fronted Warbler and Audubon's Warbler requires further study to demonstrate that they freely interbreed and should be considered conspecific. In addition, Audubon's Warbler may be a hybrid species that was created by interbreeding beween the Black-fronted Warbler and Myrtle Warbler (Milá et al 2011)
Brelsford and Irwin (2010) focus on the hybridizing populations of Audubon’s and Myrtle Warblers. These two well-known wood warblers do not interbreed freely, but maintain their identity in part to postzygotic selection against hybrids. Genetic similarities are due to incomplete lineage sorting rather than to gene flow. They are biological species. Brelsford and Irwin summarize their conclusions on page 3058:
“Our findings indicate that the myrtle and Audubon’s warblers are stable and genetically distinct forms, that parts of their genomes remain distinct despite extensive hybridization, and that selection maintains differences between the taxa. We therefore suggest that these two taxa may meet the current criterion for full species status between hybridizing North American birds, that the hybrid zone be “narrow and stable” (AOU 1998).”
The full abstract of this important paper, and the paper itself are available at http://www3.interscience.wiley.com/journal/122514080/abstract?. The paper includes an excellent review of the taxonomic history as well as the new data.
American Ornithologists’ Union. 1973. Thirty-second supplement to the American Ornithologists’ Union (AOU) check-list of North American birds. Auk 90:411–419.
Brelsford A and DE Irwin. 2009. Incipient speciation despite little assortative mating: the Yellow-rumped Warbler hybrid zone. Evolution 63: 3050-3060.
Milá, B, TB Smith, & RK Wayne. 2007. Speciation and rapid phenotypic differentiation in the Yellow-rumped Warbler Dendroica coronata complex. Molecular Ecology 16:159-173.
Milá, B., D. P. L. Toews, T. B. Smith, and R. K. Wayne. 2011. A cryptic contact zone between divergent mitochondrial DNA lineages in southwestern North America supports past introgressive hybridization in the yellow-rumped warbler complex (Aves: Dendroica coronata). Biological Journal of the Linnean Society 103:696–706.