Systematics
More than 1 species may exist within Warbling Vireo as currently classified (Am. Ornithol. Union 1998). Two species (Sibley and Monroe 1990) or even 3 species (Sibley 1996) sometimes recognized. Differences in morphology (especially overall size and size of bill), possibly voice (J. Barlow and B. McGillivray unpubl.), genetics (Johnson et al. 1988, Murray et al. 1994), and ecology (differing Prebasic molt strategies described by Voelker and Rohwer [1998]; subtle differences in breeding habitat noted in Semenchuk 1992; and different wintering areas). More study needed, however, from across range, not just areas of potential overlap (Alberta), and from disjunct populations (s. Baja and some mainland Mexico breeders poorly known), and closely related taxa (e.g., Brown-capped Vireo). Studies where Vireo gilvus swainsoni and nominate V. g. gilvus co-occur in Alberta (J. C. Barlow pers. comm.) and Montana (A. J. Banks pers. comm.) are ongoing.
Geographic Variation
Populations in western North America (se. Alaska, s. Yukon Territory, n. Alberta south through Rocky Mtns.) have slighter bill that is less swollen (smaller average depth and width) and more extensively dark upper mandible than eastern populations (central Alberta south and east). Isolated breeding population in extreme s. Baja (V. g. victoriae), however, approaches eastern birds (nominate gilvus) in this swelling of bill. Western North American birds also average about 25% smaller in body size (but with relatively longer tail) and tend to show more contrast between crown and rest of upperparts, which tend to be more tinged olive green, and show less yellow on underparts than eastern populations. Song said to differ where eastern and western populations meet in central Alberta, western birds tending to have less musical, more choppy song (higher tones and more breaks and peaks; see Sounds: vocalizations, below); uncertain whether these differences consistent among all western and eastern birds, and variation exists within each region. In the West, Warbling Vireos of Rocky Mtns. largest, with darker crowns and duller upperparts, compared with those to the south and west, where crown sometimes not particularly dark. Some southern birds of eastern population (nominate gilvus) may have less swollen bills than those in north. Above mainly based on Phillips 1991 (see for details).
Information on genetic variation, based on very limited sample sizes, show differences across range. Allozyme data found Nei's genetic distances between 0.103 and 0.076 between samples of V. g. swainsoni, "V. g. leucopolius," and V. g. gilvus (Johnson et al. 1988). Analysis of mitochondrial DNA (cytochrome b gene) from Alberta of swainsonii and nominate gilvus showed about 3% sequence divergence (Murray et al. 1994).
Subspecies
Most recent treatment is by Phillips (1991), who recognized 5 subspecies tentatively divided into 2 species. Classification by Blake (1968) recognized 6 subspecies within Warbling Vireo (excluding Brown-capped Vireo taxa lumped by this author under Warbling Vireo). Subspecies divided into 2 groups by Am. Ornithol. Union (1998), and this arrangement followed here, using taxa recognized by Phillips (1991). Although there are differences between eastern and western groups, Phillips (1991) stated that swainsonii is dubiously distinct at the species level, noting evidence of hybridization (Banff, Alberta) and that no single morphological character distinguishes all gilvus group from swainsonii group.
Western Warbling-Vireo (Swainsoni Group).
Small-billed, more extensively dark on upper mandible, small body size (see Measurements, below), generally more olive green on upperparts and darker gray on crown, Prebasic molt completed after leaving breeding grounds; winters in Mexico. ...
Eastern Warbling-Vireo (Gilvus Group).
Bill averages wider and deeper (more swollen), upper mandible with more extensive pale area, body size larger, upperparts duller and grayer (including crown, which differs only slightly from back), completes Prebasic molt on breeding grounds; winters in Central America (south of Isthmus of Tehuantepec) from s.-central Chiapas south to Nicaragua. ...
