Editor’s Note (August 2016): Maps, rich media, and text have been updated to reflect a taxonomic change/split for this species. This species account is still being edited and may contain content from an earlier version of the account.
Probably the most abundant loon in North America, the Pacific Loon is strictly marine except during its brief three-month breeding season when it nests on freshwater ponds throughout much of the arctic and subarctic tundra and taiga regions of the continent. During short periods in the spring, and to a lesser extent in fall, it is a spectacularly abundant and conspicuous migrant along the Pacific Coast. During the rest of the year it is a comparatively inconspicuous member of the North American avifauna because of its preference for arctic breeding ponds and Mexican wintering waters.
The breeding biology of this loon has been studied extensively, especially in comparison to the sympatric Red-throated Loon (Gavia stellata), with an eye toward elucidating patterns of habitat partitioning by these two species. Important works referenced frequently throughout this account include studies: (1) on the coastal plain of western Hudson Bay at the mouth of the McConnell River, Nunavut, Canada (60°50'N, 94°25'W; 3 years of field work 1967–1969, Davis 1972b ); (2) at Storkersen Point, on the Beaufort Sea coastal plain of Alaska (70°25'N, 148°15'W; 5 years of field work 1971–1975, Bergman and Derksen 1977 ); and (3) on the Yukon-Kuskokwim River Delta (hereafter Y-K Delta), 24 km from the Bering Sea coast near Hooper Bay, Alaska (61°26'N, 165°26'W; 2 years of field work 1974–1975, Petersen Petersen 1976c , Petersen 1979b , Petersen 1989 ). First described by George N. Lawrence in 1858, the nomenclatural history of the Pacific Loon is closely linked with its closest relative, the Arctic Loon (Gavia arctica), with which it has been considered conspecific at various times in the past. It is now considered a distinct species (Am. Ornithol. Union American Ornithologists' Union 1985 ), based on sympatric breeding in northeast Asia and western Alaska. Because these two species were combined for much of the 1900s, most literature on the Pacific Loon was published under the name Arctic Loon (as G. a. pacifica). Further complicating matters, the Arctic Loon is comprised of two subspecies, a widespread Eurasian race, G. a. arctica, to which most older studies pertain, and a Siberian race, Green-throated Loon (G. a. viridigularis), whose breeding distribution extends eastward marginally to western Alaska.
Although many studies have examined the ecology of the Arctic Loon across much of Eurasia, most studies pertain to European and western Asian populations, with those breeding in northeastern Siberia and western Alaska remaining virtually unstudied, making it one of the least studied regular breeding species in North America. This account summarizes available information on the North American subspecies, the Green-throated Loon, which is the only race known to occur (based on specimens). Also included is selected information from studies of Eurasian populations that may be applicable to the North American population. For details of the more extensive work on Arctic Loon across Eurasia, readers should consult Witherby et al. 1939 , Lehtonen 1970 , Cramp and Simmons 1977 , Sjölander 1978 , and Del Hoyo et al. 1992 .