The Common Loon (Gavia immer) is a large, migratory waterbird whose haunting calls are often considered the sound of the northern wilderness. These birds breed on freshwater lakes across Canada and the northern United States. While the belief that loons pair up for life is widespread, the reality of their mating system is more nuanced. Their long-term association is driven less by partner loyalty and more by a strong attachment to their breeding territory.
Site Fidelity Versus Partner Fidelity
Common Loons are serially monogamous, meaning a male and female stay together for a single breeding season, often reuniting for many seasons thereafter. Pair bonds have been documented to last, on average, for six to seven years, and sometimes for a decade or more. This long-term association is not primarily due to devotion to the partner, but rather to extreme loyalty to the breeding site, a behavior known as site fidelity.
Loons exhibit high site fidelity, with both sexes tending to return to the exact same lake territory year after year. If both members of the previous pair survive the winter and return to their established territory, they will almost certainly pair up again. The site itself, with its proven history of successful nesting, is a far greater predictor of pairing than any bond between the two individuals.
Maintaining and Defending Territory
A quality breeding territory is indispensable for reproductive success. These territories, which can encompass an entire small lake or a section of a larger one, are jointly defended by the established pair. The male is especially dedicated to territory defense, often returning first in the spring to reclaim the site.
Loons use a variety of aggressive vocalizations and displays to deter intruders. The male’s territorial yodel, a long, complex call, communicates his size, condition, and willingness to fight. Both loons also use the tremolo call, a wavering sound that signals alarm or distress when disturbed by a rival or other threat. Maintaining an established territory is advantageous, as it avoids the high energy cost and risk associated with finding and securing a new breeding site.
Causes of Pair Dissolution
A loon pair bond typically ends in one of two major ways: the death of one partner or an aggressive territorial takeover. The most common cause of dissolution is the mortality of one member during migration or on the wintering grounds. When the surviving loon returns to the breeding site, it quickly accepts a new mate from the pool of non-breeding loons, called floaters, to maintain its hold on the territory.
The second significant cause is territorial displacement by a younger, more aggressive intruder of the same sex. An intruding male will actively fight the resident male for control of the lake, battles that can be fatal for the resident loon in nearly one-third of confrontations. If the intruder successfully evicts the resident male, he will then pair with the established female, and the territory remains intact. True “divorce,” where both partners return but choose not to pair up, is rare but sometimes occurs after multiple seasons of failed nesting attempts.