Extensive hybridization in a contact zone between MacGillivray's warblersOporornistolmieiand mourning warblersO. philadelphiadetected using molecular and morphological analyses

Abstract

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).

Genetic Introgression: An Integral but neglected component of speciation in birds

“…A large combination of marker types may be needed to identify individuals with introgressed DNA (Väli et al 2010b). Hybrid zones are often "tension zones" in which selection acts against the neutral diffusion of alleles across species (e.g., Kvist and Rytkönen 2006;Carling and Brumfield 2008;Gay et al 2008Gay et al , 2009Irwin et al 2009a;Mettler and Spellman 2009). Given constant ecological or reproductive conditions, a hybrid zone can be constrained to a narrow width by this mechanism, preserving the genetic integrity of the species on either side.…”

Morphological expression in putative intergrades between two subspecies of Orange-crowned Warbler (Oreothlypis celata) on the Kenai Peninsula, Alaska

“…Hybrid zones can be relatively wide and dominated by morphological and genetic intergrades between the parental taxa (e.g., Sibley and Short 1964, Irwin et al 2009, Vallender et al 2009, Den Hartog et al 2010, or relatively narrow and dominated by individuals that are morphologically and genetically close to the parental taxa (e.g., Barrowclough et al 2005, Leafloor et al 2013. At the possible extreme, one study found little evidence for interbreeding between two sympatric and closely-related passerine species, and possible hybridization only can be speculated (Toews and Irwin 2008).…”

A primer on the hybrid zone of Jamaican streamertail hummingbirds (Trochilidae:Trochilus)

“…Variance in cline width has been used to infer selection coefficients associated with particular characters (Barton & Hewitt 1985). The majority of avian hybrid zones studied thus far exhibit cline widths of 100-1000 km (Barrowclough 1980, Rohwer et al 2001, Irwin et al 2009, Mettler & Spellman 2009. Case examples of narrow avian hybrid zones with cline widths of 10-100 km have been documented in manakins (Brumfield et al 2001), wrens (Selander 1965), crows (Meise 1928), titmice (Curry & Patten 2014), warblers (Toews et al 2011), and grackles (Yang & Selander 1968).…”