Cryptic speciation in the Warbling Vireo (Vireo gilvus)

Eastern (Vireo gilvus gilvus) and  western (V. g. swainsoni)
forms of the Warbling Vireo have essentially allopatric breeding ranges
across north-central North America, but come into contact in central
Alberta, Canada.  In 1986, Jon Barlow presented preliminary
morphological and song evidence suggesting that the Warbling Vireo complex
might comprise more than one valid species.  However, to date
Barlow’s suggestion is supported by only limited DNA evidence,
demonstration of molt and migration differences between the taxa, and
anecdotal accounts of differences in song, morphology, plumage, and
ecology.  We analyzed variation in both mitochondrial and nuclear
DNA in birds from Alberta and surrounding areas to determine the levels of
genetic differentiation and  hybridization occurring in the
contact zone, and whether the two taxa warrant recognition as separate
biological species.  Our analyses reveal that Warbling Vireos in
Alberta and the surrounding areas are separated into two well-defined,
genetically-differentiated, and monophyletic clades corresponding to
previously-recognized taxonomic groups.  The two taxa come into
contact in a narrow (~85 km) zone in Barrhead County, northwest of
Edmonton, Alberta.  They show evidence of limited
hybridization.  The distinct genetic differences are maintained
in the contact zone, where individuals of the two taxa may occupy
neighboring territories.  Differences in spring arrival dates,
molt schedules and migration routes indicate that a migratory divide may
play an important role in reproductive isolation.  We suggest
that the two taxa are distinct cryptic species: an eastern form, Vireo
gilvus, and a western form, Vireo swainsoni.