Refuge Notebook 07/08/2011

Do Common Loons Mate for Life? New Research Provides Answers
by Ted Bailey

As this year’s Common Loon chicks appear on local lakes in late June and early July, you might still assume that loons mate for life. However, a recent summary of 18 years of research on individually marked loons on lakes in Wisconsin revealed new information on loon mating systems, territory acquisition by young adults, territorial defense by established residents and the function of vocalizations. The article “Marking Loons, Making Progress” by researchers Walter Piper, Jay Mager and Charles Walcott from Chapman University, Ohio Northern University and Cornell University, respectively, appeared in the May-June 2011 issue of American Scientist. Such comprehensive behavior information was not available until 1992 when David Evers, another researcher of loons, discovered how to efficiently capture and mark individual loons. David, Liz Jozwiak (a former refuge biologist) and I were the first to capture and mark loons on the Kenai National Wildlife Refuge in 1995 as part of David’s ongoing research on loons.

The findings from the Wisconsin research revealed – in contrast to previous assumptions - that loon pairs do not mate for life. Significant findings of their study were that; 1) males or females of territorial loon pairs routinely vanish from their territories leaving one marked and the other unmarked loon in their place; 2) there were regular evictions of territorial owners; 3) “floaters” (male and female loons lacking territories but resembling territory holders) often visited established loon territories an average of two to five times per day and stayed five to thirty minutes interacting with the resident loons; 4) unlike many other birds, male floaters did not secretly mate with the territorial female (confirmed by DNA fingerprinting); 5) recently successful territories that produced chicks were especially targeted by floaters apparently because the presence of chicks is an indicator of a desirable high quality territory; and 6) one-third of all battles between male loons resulted in the death of the territorial male loon because territorial males often fought to their death with the intruding male.

It was also discovered that territorial male loons apparently select the nest site on a lake and, if it is successful, the pair of loons use the same nest site in subsequent years until the male is evicted from the territory. If unsuccessful, a different nest site is chosen. But the highly successful territories were the ones most sought after by intruding floater loons. It was further learned that although young (3 to 4 years old) males almost always acquire a territory by settling in a vacant lake, older (5 to 6 years old) males will suddenly begin to try to take over established territories. Keep in mind that loons may live to be over 20 years old!

Intruding male loons apparently pay attention to the calls (yodels) given by the defending resident males to assess their chances of usurping them in combat. At their peak performance, territorial male loons are at their greatest body mass and their yodels are given at low frequencies. A higher pitched yodels means a resident male may be less fit or has lost body mass – perhaps because of old age, sickness or injury - meaning the chances of another male taking over his territory are increased and lethal combat may occur.

I once watched two Common Loons on a lake continuously battle each other; they were still chasing each other and fighting 45 minutes later when I left. I also once found a dead loon on another lake whose breast had been pierced by the sharp bill of another loon. So in contrast to the apparent serene lives we often see loons living, it seems that dramatic changes occur during the lifetime of territorial male loons and their lives can end in sudden and lethal violence. Summarizing, the Wisconsin study has shown that territorial takeovers are common among loons but only males in prime condition achieve such takeovers; males also control where the nest is located, and his territorial yodeling betray his size and aggressive motivation. Enjoy watching loons this summer but with the knowledge that their lives are not always one of peace and tranquility.

Ted Bailey is a retired Kenai National Wildlife Refuge wildlife biologist who has lived on the Kenai Peninsula for over 35 years. He maintains a keen interest in the Kenai Peninsula's wildlife and natural history. Check out our Facebook at