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Loons (Gaviidae) divide their annual cycle between salt and freshwater habitat, the latter being the site of breeding activites. Territorial requirements include sufficient size to permit runs of more than 100 m for take-off and landings; food for chicks and water clarity sufficient to permit foraging for it at depths generally < 5 m; protected nest sites, preferably islands; and shallow vegetated nurseries. Gavia stellata, the smallest and most primitive species, is the exception, and breeds on small tarns, requires take-offs of < 50 m, and flies to coastal or riverine waters for all food. Freshwater dependence has increased during loon evolution, shaped their vocalizations, and mandated annual migrations. Hazards include eutrophication, human disturbance, water level fluctuations, toxic pollutants, acid precipation and seasonal unreliability. Loons have long been considered a major indicator of wilderness quality, but their potential as useful monitors of freshwater habitat depends on availability of baseline data, which currently varies among species and geographical regions.

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