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 association with Arctic breeding ponds and overwintering in Mexican waters. The breeding biology of the Pacific 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 species. Important works referenced frequently throughout this account include studies on the coastal plain of western Hudson Bay at the mouth of the McConnell River in Nunavut, Canada (1967–1969) (1); at Storkersen Point, on the Beaufort Sea coastal plain of Alaska (1971–1975) (2); and on the Yukon–Kuskokwim Delta, 24 km from the Bering Sea coast near Hooper Bay, Alaska (1974–1975) (3, 4, 5).
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. Since 1985, the American Ornithologists' Union has recognized the Pacific Loon and Arctic Loon as distinct species (6), based in part on sympatric breeding in northeastern Asia and western Alaska. However, because these species were combined for much of the 1900s, most of the 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, the "Green-throated" Loon (G. a. viridigularis), whose breeding distribution extends marginally eastward to western Alaska. Information on the Pacific Loon in this account will focus on North American populations. Details on Siberian populations can be found in Dement'ev and Gladkov (7) and Portenko (8).