Loons are the only surviving members of an ancient order of birds, the Gaviiformes, which has a fossil record extending back to the Lower Cretaceous, more than 100 million years ago. Loons comprise their own family, the Gaviidae, which consists of 12 extinct and five surviving (extant) species. All of the extant species of loons live in the Northern Hemisphere, where they breed on lakes and ponds, from the northern part of the temperate zone to the high arctic, and winter on marine waters, near the shore, mostly in temperate and boreal climates. All of the loons have,
to varying degrees, a Holarctic distribution, meaning they occur throughout most of the Northern Hemisphere, in both Eurasia and North America. In Eurasia loons are called divers.
All loons have heavy, waterproof plumage, generally colored black or grayish above and white below. During the breeding season, the above-water plumage can be strikingly marked in a species-specific fashion, but the sexes are not distinct. During winter, the plumage is much more plainly countershaded (that is, dark above and white below). Loons are strong, direct flyers, and are capable of daily long-distance trips between feeding and nesting habitats. They also undertake longer seasonal migrations, during which they may travel more than 3,100 mi (5,000 km). However, because they are heavy birds with relatively small wings, loons must run briefly over the surface of the water to gain enough speed to become airborne (with the exception of the relatively small, red-throated loon, which can take off directly). During the feather molt, loons cannot fly at all. Usually, for safety, the molt is carried out on a large body of water.
Loons are excellent swimmers, propelling themselves with large webbed feet, located relatively far to the rear of the body in order to make swimming more efficient. To some degree, loons use their wings while swimming, but underwater the wings are used mostly for steering. Because of the rearward placement of their legs, loons are very clumsy and almost immobile on land. Consequently, their nests are placed close to the shoreline, preferably on an island or islet, and not elevated much above the surface of the water. Loons typically lay two eggs in a crude nest, essentially a scrape. Both sexes participate in the incubation and rearing of the young. Loon hatchlings can swim almost immediately, but it takes almost two months before they are capable of flying. While they are small, the babies often roost within the back feathers of a parent. Loons typically mature at three years of age. Immature birds stay in marine waters until they are ready to breed.
Loons mostly eat small fish, which they seize underwater with their bill. Loons may also consume frogs and larger species of aquatic crustaceans and insects, especially if they are breeding on fishless ponds. In such situations, loons may also travel from the breeding pond to the ocean or to a larger lake with fish in search of food.
Loons are known for their extraordinarily haunting and resonant calls and wails, which may be heard while they are flying or while they are on the water. In addition, they often engage in spectacular courtship and territorial displays while running over the water, sometimes while calling. Loons do not call during winter.
The red-throated loon (or red-throated diver, Gavia stellata ) has a wide, Holarctic distribution, breeding on Arctic and subarctic lakes and ponds in both Eurasia and North America, as far north as the limit of land on the high Arctic islands and Greenland. It is the smallest loon, and the only one capable of taking off directly from water and from land. Consequently, the red-throated loon can breed on smaller ponds than any other species of loon.
The Arctic loon (black-throated diver, G. arctica ) is found in eastern Eurasia and western Alaska and nearby Canada, and is closely related to the Pacific loon (G. pacifica ), which occurs more widely in northern North America. In fact, until recently these were considered to be a single, Holarctic species, under Gavia arctica.
The common loon (great northern diver, G. immer ) breeds in subarctic and northern temperate regions of North America, as far south as the Great Lakes region. The common and yellow-billed loons tend to replace each other geographically and ecologically, with little overlap in their distributions. The yellow-billed loon (white-billed diver, G. adamsii ) is more common in Eurasia, especially Siberia, and in extreme northwestern North America.
In the past, loons have been killed by humans in some areas because they were viewed as important competitors for fish. Loons have sometimes been hunted for their feathers and skins, but are rarely eaten because of the strong, fishy taste of their flesh. Today, loons are threatened by oil spills in their
oceanic wintering habitat, by tangling in fishing nets, by deforestation and other habitat damage in the surroundings and edges of breeding lakes, by cottage development and motorboats, and by the effects of acid rain and aquatic mercury pollution. Acid rain can acidify lakes and ponds in the northern breeding range of loons, causing these bodies of waters to lose their fish population and exposing the birds to toxic elements such as mercury, cadmium, and aluminum.
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