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Systematics
Geographic Variation
A geographic cline in size is well established: individuals from interior breeding populations, in the upper Great Lakes and central Canada, are smallest, and size increases to the east and west from there (Rand 1947, Anderson et al. 1970, Storer 1988). Field measurements of over 2,400 breeding adult loons, representing breeding populations in 10 states and 7 provinces, confirm this pattern (DCE, Appendix I). Yodel frequency also follows a geographic cline, with higher frequency vocalizations from interior birds and lower frequency vocalizations from coastal birds (Mager et al. 2007b). No variation in plumage or coloration has been described.
An incomplete genetic profile across North America indicates there are small but significant genetic differences between eastern populations (east of the Great Lakes including Quebec and New York) vs. central populations. Birds from the central portion of North America and west differ little genetically (A. McMillan, pers. com.), which may indicate that there are barriers to movement among areas in the eastern portion of North America that could result in continued differentiation.
Subspecies
Monotypic. Despite broad variation in multiple morphometric characters, there is substantial individual variation, and geographic variation is smoothly clinal. Thus, the supposedly smaller G. i. elasson Bishop, 1921, breeding in western and central North America is not a valid subspecies (Rand 1947, Storer 1988).
Related Species
Most closely related to the Yellow-billed Loon in morphology, nDNA, mtDNA, and vocal repertoire (Lindsey 2002) and thought to constitute a superspecies with it. These two species are likely closely related to the Pacific Loon, with the Arctic Loon and Red-throated Loon more distant (Boertmann 1990, Lindsey 2002). Hybrids with young have been observed/reported with Yellow-billed, Pacific and Arctic loons (JWM/JFB, Robertson and Fraker 1974, Hunter and Dennis 1972). The relationship between Pacific and Arctic loons as a sister clade is questioned (Lindsey 2002).
Evers, David C., James D. Paruk, Judith W. Mcintyre and Jack F. Barr. 2010. Common Loon (Gavia immer), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/313
