J. Avian Biol. 40: 539552, 2009 doi: 10.1111/j.1600-048X.2009.04687.x # 2009 The Authors. J. Compilation # 2009 J. Avian Biol. Received 15 August 2008, accepted 7 January 2009 Extensive hybridization in a contact zone between MacGillivray’s warblers Oporornis tolmiei and mourning warblers O. philadelphia detected using molecular and morphological analyses Darren E. Irwin, Alan Brelsford, David P. L. Toews, Christie MacDonald and Mark Phinney D. E. Irwin (correspondence), A. Brelsford, D. P. L. Toews and C. MacDonald, Biodiv. Res. Centre, and Dept. of Zool., Univ. of British Columbia, 6270 Univ. Blvd., Vancouver, BC, V6T 1Z4, Canada. E-mail: irwin@zoology.ubc.ca.  M. Phinney, Louisiana-Pacific Canada Ltd., 116-116th Ave, Dawson Creek, BC, V1G 3C8, Canada. There are many pairs of related western and eastern avian taxa in North America, and for many of these, little is known about their interactions in sympatry. One example is provided by MacGillivray’s warblers Oporornis tolmiei and mourning warblers Oporornis philadelphia. There have been occasional reports of range contact and hybridization between these forms, but recent authors have doubted these reports. We show that these two species do in fact come into extensive range contact in the southern Peace Region of British Columbia, just east of the Rocky Mountains. We analyze whether patterns of variation in morphometric traits, eye-arcs, a mitochondrial DNA marker (COI), and a Z- chromosome marker (CHD1Z) are consistent with reproductive isolation or hybridization in this contact zone. Each trait shows strong differences between allopatric MacGillivray’s warblers and allopatric mourning warblers, yet in the contact zone there are many birds with a combination of traits typical of both species. This is clearly seen in the molecular markers, for which 18 of 50 birds genotyped in the contact zone have both western and eastern alleles. These patterns strongly indicate the presence of an extensive hybrid zone between MacGillivray’s and mourning warblers. Variation in each of the four traits is explained well by a single sigmoidal cline, with a width of roughly 150 km (or 130 km based only on the molecular markers). This is only the fourth hybrid zone known among North American wood-warblers (Parulidae). The study of reproductive isolation has been a central component of the growing body of research on speciation (Coyne and Orr 2004, Price 2008). To test whether two groups are reproductively isolated, it is important whenever possible to study the two groups in a geographic region where they both occur. We can then determine whether they differ in various traits in a similar environment, how they interact, and whether they interbreed. These observa- tions can help us to reconstruct the evolutionary history of the two groups, to understand their current interactions, and to more reliably predict their future. Such knowledge enables better decisions regarding taxonomic treatment and the conservation of biodiversity. In North America, there are many examples of western and eastern avian taxa that are apparently close relatives (Newton 2003, Weir and Schluter 2004). In some cases, the two taxa have been considered subspecies of a single species (e.g. winter wrens Troglodytes troglodytes pacificus and T. t. hiemalis, Hejl et al. 2002, Drovetski et al. 2004, Toews and Irwin 2008; yellow-rumped warblers Dendroica coronata auduboni and D. c. coronata, Hunt and Flaspohler 1998, Mila ´ et al. 2007; northern flickers Colaptes auratus cafer and C. a. auratus, Moore 1995), whereas in other cases the western and eastern taxa have been considered two species (e.g. lazuli and indigo buntings Passerina amoena and P. cyanea, Greene et al. 1996, Payne 2006, Carling and Brumfield 2008; MacGillivray’s and mourning warblers Oporornis tolmiei and O. philadelphia, Pitocchelli 1993, 1995; spotted and eastern towhees Pipilo maculatus and P. erythrophthalmus, Greenlaw 1996). In some such cases, little research has occurred in areas of potential contact between the western and eastern relatives. Research from such contact areas, if they exist, could provide surprising results. For example, recent work in a contact zone between western and eastern forms of winter wrens Troglodytes troglodytes showed a high level of reproductive isolation, along with behavioral and genetic divergence, indicating that they are separate species that have been evolving relatively independently for millions of years (Toews and Irwin 2008; see also Drovetski et al. 2004). Here, we examine a case of two closely related taxa that have been considered separate species since they were first described: MacGillivray’s warbler Oporornis tolmiei (described by Townsend in 1839) in the west and mourn- ing warbler Oporornis philadelphia (described by Wilson in 1810) in the east (Pitocchelli 1993, 1995, Dunn and 539