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"...the wild call of the Red-throated Diver is enough to make one's flesh creep: it resembles the cry of a little child in pain, but is more akin maybe to the wail of a lost spirit, echoing and re-echoing round the lonely hills." E. L. Turner, 1913 (1)
Loons typically evoke images of broad, picturesque lakes, striking black and white breeding plumage, and melodic territorial yodels. The Red-throated Loon does not fit this archetype. Instead, this smallest member of the family Gaviidae breeds in small arctic and boreal lakes, bears relatively plain gray plumage, save for its namesake throat patch, and utters a call during breeding that is generously described as cacophonous. In further contrast to congeners, the Red-throated Loon does not feed within its territorial breeding lake, but instead behaves as a central-place forager, flying between separate breeding and foraging areas, often inshore marine habitat, to feed and to provision its young. Rather than molting flight feathers during winter, as do other loon species, the Red-throated Loon molts during fall, often at sites intermediate to breeding and wintering sites. Smaller body size allows it to take flight over a much shorter distance, perhaps an adaptation to an earlier period in its evolutionary history when most ice-free lakes available for breeding were small and shallow.
With a circumpolar distribution, the Red-throated Loon breeds in tundra and boreal landscapes, up to high latitudes. During winter, it primarily resides along both Atlantic and Pacific coasts of North America and Eurasia, the Great Lakes, and Black, Caspian and Mediterranean Seas. In North America, it breeds mainly on small ponds in remote coastal tundra habitat, so it is not often encountered while in its breeding plumage. In contrast, the Red-throated “Diver” is better known in Eurasia, where it breeds in more populated regions, including Scotland and Scandinavia. In the most northern parts of its range, above 75°N, it has only 2–3 months to nest and raise its young to fledging.
One of the earliest publications on avian behavior, and the first major work on any loon species, was J. S. Huxley's (2) landmark treatise on Red-throated Loon behavior and the evolution of courtship in birds. For decades, the species has been the subject of considerable field research to monitor reproductive success in Sweden (3, 4, 5, 6) and the United Kingdom (7, 8, 9). Individuals banded during these efforts have provided information on dispersal, migration routes, and survival rates ( 10, 11). Foraging behavior, chick diet, and characteristics of boreal breeding habitat have been quantified (12, 13, 14), specifically in relation to lake acidification by airborne contaminants from fossil-fuel-burning power plants (15, 16, 17, 18) and the consequent degradation of breeding habitat (16) and contamination of eggs (19, 20).
In North America, Red-throated Loon populations have varied widely in several parts of the breeding range, perhaps due to variations in ocean conditions, such as water temperature and its effects on marine fish communities (21, 22). This population variation has stimulated an increase in research since 2000 in North America, where aspects of its breeding biology have been investigated at sites from Alaska (23, 24, 25, 26), across the Canadian Arctic (27, 28, 29, 30, 31), and towards the southern extent of its range on Haida Gwaii in British Columbia (32, 33), often in comparison to sympatric Pacific Loon (Gavia pacifica; 34, 35, 36, 37). Satellite telemetry has provided valuable information on migratory movements of breeders from Alaska (38), and movements and marine habitat selection of loons wintering along the mid-Atlantic coast (39). Its dispersed distribution across large, remote terrestrial and marine habitats has made quantifying demographic parameters, other than the fate of eggs, challenging; there is only one study that has estimated adult survival in North America (22), and key parameters, such as juvenile survival and age of first breeding, are unknown.
Because of its sensitivity to disturbance, broad distribution in coastal tundra habitat, and reliance on the marine environment during breeding, the Red-throated Loon has been identified as an indicator species for environmental change on its breeding habitat (36, 40, 16). This role is likely to become more important given the unprecedented changes occurring in both coastal tundra and inshore marine habitats as the average global temperature rises in response to anthropogenic greenhouse gas emissions.