Gavia immer

Considered conspecific with G. adamsii by some authors (AOU 1983).

Still a common species in many portions of its large range but perhaps re-evaluate periodically as monitoring efforts improve.

Nesting occurs in Iceland, Greenland, and across Canada and the northern United States to Alaska, and south to California, Montana, the Great Lakes region, New York, New England, and Nova Scotia (AOU 1998). In winter, this loon occurs mainly along the Pacific coast from Aleutians to Baja California and Sonora, along the Atlantic and Gulf coasts from Newfoundland to Florida and west to Texas, and in the western Palearctic along the Atlantic coast to northwestern Africa (AOU 1998). In North America, this species is most concentrated in winter along the South Carolina coast, around Vancouver Island, in northern California, along the Gulf Coast adjacent to the Florida panhandle, and along the Atlantic seaboard from Massachusetts to Maine (Root 1988).

This species is represented by a large number of occurrences.

Susceptible to human disturbance at breeding lakes (via development of shoreline areas and aquatic recreational activities), acid rain alterations of lake ecosystems, and mercury poisoning (USFWS 1987, Rimmer 1992; St. John, 1993). Also may be jeopardized in some areas by fluctuating water levels at the nest site and by increasing numbers of predators such as raccoons (Rimmer 1992).

HABITAT LOSS AND DEGRADATION: Direct and indirect effects of shoreline development may reduce the suitability of lakes for nesting. Although radical shoreline alteration and cottage construction appear to only rarely inhibit nest site selection, increased human activity around developments often does (McIntyre 1988, Sutcliffe 1980, Zimmer 1979). Hatching success decreased as the number of cottages within 150 meters of nests increased on lakes in central Ontario (Heimberger et al. 1983). McIntyre (1988) found that the number of lakes with territorial loons decreased with increasing shoreline development and recreational use in Minnesota between 1971-86. Water-level fluctuations resulting from human-made dams can also reduce the suitability of a lake for breeding. Fair (1979) documented nest abandonment and predation following lake drawdowns in New Hampshire. On Stillwater Reservoir in New York, McIntyre (1988) found that nests were lost to inundation when water levels rose more than 20 cm. Breeding loons in an area of regulated water levels in Minnesota had significantly lower productivity than other populations on naturally fluctuating lake systems (Reiser 1988). Nesting may be delayed by water levels that fail to recede after snowmelt (Fair 1979, Strong 1985). Although poorly regulated lake levels can lead to nest losses, creation of reservoirs has increased the availability of suitable nesting habitat in some areas (e.g., McIntyre 1988).

HUMAN DISTURBANCE AND HUNTING: Recreational pressures may have contributed to declines in some populations, but loons generally can acclimate to moderate recreational lake use. While Ream (1976) suggested that disturbance of nest sites by canoeists in the Boundary Waters Canoe Area in Minnesota was the primary factor limiting productivity, Titus and VanDruff (1981) later found few negative impacts from recreational activities in the Boundary Waters Canoe Area. Smith (1981) reported identical productivity on both remote lakes and on lakes with established canoe routes in Alaska. In Maine, no significant difference in breeding success was found for loons on high human-use versus low human-use lakes (Christenson 1981). However, densities may be lower on heavily developed than on relatively undeveloped lakes (McIntyre 1988).

When incubating loons leave nests because of disturbance, they may not return for an hour or more, leaving the eggs vulnerable to predation and cooling (McIntyre 1975, Titus and VanDruff 1981). Loons exhibit behavioral modifications in response to moderate recreational activity on many lakes. On high human-use lakes, loons flush at shorter distances (Smith 1981, Titus and VanDruff 1981), flush less readily and less vigorously, vocalize less once flushed, and return to the nest more quickly than loons on remote lakes (Titus and VanDruff 1981).

Motorboats may impact loons more negatively than canoes due to differences in peak use and breeding periods. On Boundary Waters Canoe Area lakes, motorboat use is heaviest early in the season, when loons are nesting, while canoe use peaks in August after the nesting season (McIntyre 1988). Loons are more easily able to avoid canoes than motorboats, and chicks are less likely to be separated from their parents by canoes. Motorboat wakes in combination with high water levels may cause nest destruction (Vermeer 1973).

Although the sport shooting that impacted populations around 1900 is now illegal, loons continue to be intentionally killed on occasion, primarily by sport and commercial fishers who consider the birds to be direct competitors (McIntyre 1988). Of 29 dead loons necropsied in New York from 1972-86, three had been shot (Okoniewiski and Stone 1987). Loons are still taken for food by American Indians and Inuits. In northern Quebec, the annual harvest ranges from 2,500-6,500 loons, most of them common loons (Desgranges and Laporte 1979). This harvest is thought to be too high to support the region's current population of 12,000 pairs.

COMPETITION: Intraspecific competition may limit productivity. Sibling aggression can be severe, especially during food shortages, and may result in the death of the subordinate, usually younger, chick (Dulin 1987). Chicks that wander into adjacent territories may be killed by neighboring adult loons (McIntyre 1988). Severe fighting by adults has been documented, presumably over territorial ownership, and can lead to injury or nest abandonment (e.g., Kaveney and Rimmer 1989). Competition with aggressive, non-native mute swans (CYGNUS OLOR) has been documented in Michigan (McPeek and Evers 1989).

ENTANGLEMENT: Mortality is known to occur from entanglement in monofilament sports fishing line and in commercial fishing nets (Vermeer 1973, Okoniewski and Stone 1987). Commercial fish traps and nets in the Great Lakes pose a serious, although unquantified, threat to loons (McIntyre 1986, 1988). Loons are also caught in nets used during coastal fishing operations (McIntyre 1978). Most mortality from these sources goes unreported.

ENVIRONMENTAL POLLUTANTS: Organochlorines and their residues have been detected in eggs and carcasses. DDE levels in tissue from Minnesota in the 1960s may have had adverse, sublethal effects (Ream 1976, McIntyre 1988). Eggs with higher levels of DDE residues tend to have thinner shells than eggs with lower residue levels (Vermeer 1973, McIntyre 1975, Sutcliffe 1978, Fox et al. 1980), although no studies have demonstrated evidence of shell breakage. There appears to be no documentation of lowered productivity as result of elevated pesticide loads (Fox et al. 1980), and organochlorine levels have generally declined in loon tissue in recent years (Frank et al. 1983).

Heavy metal contaminants may pose the most widespread, irreversible and deadly threat (McIntyre 1988). Methylmercury poisoning has been implicated in lowered productivity (Barr 1986) and winter mortality (Stroud and Lange 1983, Alexander 1985). Mercury is released into the environment during the operation of chlor-alkali and wood pulp plants, and through treatments of agricultural seeds (McIntyre 1988). Lake acidification may accelerate the release of mercury into the water column, hastening its uptake through the aquatic food chain (Barr 1986, McIntyre 1988). In Ontario, Barr (1986) found significantly higher mercury residues and lower successful use of territories in loons on lakes within 160 kilometers downstream of a chlor-alkali plant. High mercury levels in many necropsied loons following a large winter die-off along the Gulf Coast of Florida in 1983 (as many as 7,500 birds) (Alexander 1985) may have contributed to the emaciation and subsequent death of these individuals. In New York, Okoniewiski and Stone (1987) tentatively diagnosed mercury intoxication in three of 29 carcasses examined between 1972-86.

Other heavy metals, such as lead, cadmium and selenium, are actual or potential hazards (McIntyre 1988). Lead poisoning from ingestion of lead fishing sinkers is implicated as a cause of mortality (Locke et al. 1982, Okoniewiski and Stone 1987, Pokras and Chafel, 1992.

Acid precipitation may reduce the quality of nesting lakes. Alvo et al. (1988) monitored reproductive success on 68 small lakes (5.3-75 hectares) near Sudbury, Ontario that varied in pH from 4.0-8.4. Nesting success was higher on lakes with high alkalinity, which was negatively correlated with pH, than on low-alkalinity lakes. Unsuccessful breeding resulted primarily from brood mortalities on acidic lakes and appeared to result from lower prey fish densities. On acidified lakes in New York's Adirondack Park, chicks were fed prey much smaller or larger than those typically preferred (Parker 1988).

Oil spills pose a serious, although localized, threat to habitat. Most spills have occurred on marine wintering areas, and reports of mortality from coastal spills are common (McIntyre 1988). Spills on inland waters, particularly on staging grounds, could be disastrous to migrating loons (McIntyre 1988).

PREDATION: The major predators on nests and chicks include scavengers such as American crows (CORVUS BRACHYRHYNCHOS), common ravens (CORVUS CORAX), herring gulls (LARUS ARGENTATUS) and raccoons (PROCYON LOTOR), all of which have increased due to the proliferation of garbage dumps and other human refuse (McIntyre 1988). Raccoons caused 75-80 percent of nest losses on New Hampshire's two largest lakes in 1977 (Sutcliffe 1980). Other predators include snapping turtles (CHELYDRA SERPENTINA), northern pike (ESOX LUCIUS), muskellunge (ESOX MASQUINONGY), walleye (STIZOSTEDION VITREUM), red fox (VULPES FULVA), mink (MUSTELA VISON), skunk (MEPHITIS MEPHITIS), and occasionally other mammalian carnivores (Olson and Marshall 1952; McIntyre 1975, 1988). Otters (LUTRA CANADENSIS) and bald eagles (HALIAETUS LEUCOCEPHALUS) have been reported as possible predators (Titus and VanDruff 1981, McIntyre 1988). Harassment or disturbance by herring gulls, beavers (CASTOR CANADENSIS) and muskrats may lead to nest abandonment (Titus and VanDruff 1981). Predation on adults appears to be rare (Barklow and Chamberlain 1984, Riedman and Estes 1988).

DISEASES AND PARASITES: Susceptible to epidemics of both type C and type E botulism (McIntyre 1988). Type E botulism has killed up to 3,570 loons on Lake Michigan in a single year (Fay 1966, in McIntyre 1988), probably through ingestion of alewives during migration. Also susceptible to aspergillosis from airborne ASPERGILLUS spp. spores, which destroy the functioning of air sacs, particularly in stressed birds (Okoniewski and Stone 1987, McIntyre 1988). An extensive list of internal parasites has been documented (McIntyre 1988). Carcasses in emaciated condition from a massive die-off along the Gulf coast in 1983 had abnormally high numbers of microphallid trematodes (flukes), as well as tapeworms, spiny-headed worms, renal trematodes and renal coccidia (Stroud and Lange 1983). These parasites are believed to have caused hemorrhagic entritis and contributed to the pronounced emaciation of many dead birds. Loons are afflicted by a host-specific black fly (SIMULIUM EURYADMINICULUM), which may act as a vector and transmit a blood parasite (McIntyre 1975, 1988).