During the breeding season, loons live on lakes and other waterways in Canada, the northern United States (including Alaska), and southern parts of Greenland and Iceland.

Common loons eat a variety of animal prey including fish, crustaceans, insect larvae, molluscs, and occasionally aquatic plant life.

The United States Forest Service has designated the common loon a species of special status because of threats from habitat loss and toxic metal poisoning in its US range.

The common loon is the provincial bird of Ontario, and it appears on Canadian currency, including the one-dollar "loonie" coin and a previous series of $20 bills.

[10] The word may be related to Swedish immer and emmer: the grey or blackened ashes of a fire (referring to the loon's dark plumage); or to Latin immergo, to immerse, and immersus, submerged.

The foreneck is whitish, usually forming wedge-shaped notch in dark neck-sides,[2] and may sometimes reveal a shadowy trace of the neck ring or a pale collar.

[24] It has dark brownish grey upperparts with an unclear pattern of squares on the shoulders and some wing coverts spotted with white, which are usually concealed while swimming.

[2] The scaly juvenile plumage is retained until January or February of the year following hatching, when a lengthy moult of head and body feathers gives them a more adult-like appearance.

Adults shed all their flight feathers simultaneously around this time, leaving them temporarily flightless, prior to gaining breeding plumage, but second-year birds delay this substantial moult until the summer.

[29] Common loons are mainly Nearctic, and breed from 48° N to the Arctic Circle, locally south to 40° N and north to 78° N.[2] During their breeding season in spring and summer, most common loons live on lakes and other waterways in the northern United States and Canada, as well as in southern parts of Greenland,[30] in Iceland, in Svalbard, in Jan Mayen, and in Bear Island in Norway; and in Alaska, to the west, and very rarely in Scotland, to the east.

When there is either a lack of fish or they are difficult to catch, it preys on crustaceans, crayfish, snails, leeches, insect larvae, molluscs, frogs, annelids, and occasionally aquatic plant matter such as pondweed, roots, moss, willow shoots, seeds, and algae.

[48][49] Pairs do not remain together during winter;[2][18] in addition, males usually precede females by a few days to a few weeks during spring migration, settling on their lake once a portion of it becomes ice-free.

[51] The displays towards intruders, such as bow-jumping (an alternation of fencing and bill-dipping postures[11] and rushing (running "along the surface with its wings either folded or half-extended and flapping at about the same speed as when taking off"[52]) are often misinterpreted as courtship.

[32] The nest is about 56 cm (22 in) wide and is constructed out of dead marsh grasses and other indigenous plants, and formed into a mound along the vegetated coasts of lakes greater than 3.7 ha (9.1 acres).

As they grow, chicks are able to catch an increasingly large proportion of their diet by themselves; they can feed and fend for themselves after about two months, although many juveniles continue to beg from adults well beyond this age.

[65] Considerable information on longevity and survival rates has been collected in the past two decades, owing to the implementation of an efficient capture protocol that permits marking and monitoring of large study populations.

However, a second, finer-scaled analysis made clear that male loons begin to show higher mortality, increased territory loss and lower body condition starting at age 15.

This age-related shift in behaviour is interpreted as terminal investment, a "go for broke" strategy seen in senescing animals that are attempting to eke out another year or two of breeding before they die.

[75] The black fly Simulium annulus is closely associated with the common loon to which it is attracted to chemicals in the uropygial gland secretions as well as by visual and tactile cues.

[80] In Europe it appears in 20 Important Bird Areas (IBAs), including Ireland, Svalbard, mainland Norway, Iceland, Spain, and the United Kingdom.

[82] The main contributors to elevated mercury concentrations in aquatic environments are coal burning power plants, waste incineration, and metal production.

[83] Although environmental mercury (Hg) is naturally occurring, methylmercury (CH3Hg+, sometimes written as MeHg+) is a biologically toxic form that accumulates throughout aquatic environments in the northeastern United States.

[85] Previous research has found a correlation between mercury levels and pH, with more acidic aquatic environments being at the highest risk for elevated methylmercury concentrations.

[82] A different study carried out in the Adirondack Mountains found that elevated levels of methylmercury are associated with reduced diving frequency in adult common loons.

[84] Spatial analysis indicates that the highest aquatic mercury concentrations are found in the southwestern portion of the Adirondacks, an area with lakes heavily affected by acid rain.

[92] It too is adversely affected by acid rain and pollution, as well as lead poisoning from fishing sinkers (especially those that are about the size of the grit stones they ingest[93]) and mercury contamination from industrial waste.

[94] Heavy metals such as mercury may be partially removed through biological processes such as excretion or deposition in feathers, but their adverse effects are magnified through concentration of the toxic elements in organs such as the liver.

Some environmentalists attempt to increase nesting success by mitigating the effects of some of these threats, namely terrestrial predation and water-level fluctuations, through the deployment of rafts in the loon's breeding territories.

[97] In addition, artificial floating nesting platforms have been provided for the common loon in some lakes to reduce the impact of changing water levels due to dams and other human activities.

[111] The common loon was eaten in the Scottish Islands from the Neolithic until the eighteenth century, and its thick layer of fat beneath the skin was used as a cure for sciatica.