The loon's call consists of two parts, an introductory "phrase" followed by a highly variable number of repeat phrases.
The researchers distinguished between calls by noting changes in the frequencies of the first phrase and alterations in the gaps between the following repeat phrases.
Alec Lindsay, professor of biology at Northern Michigan University in Marquette, sees the discovery as evidence that loons are bird-brained only in the most literal sense of the term.
"For me," he said, "this study is an elegant demonstration of the complexity of a loon's brain. These animals don't just have inflexible vocalizations that they use for 25 years.
"They're doing some pretty complex interpretations of their social environment and modifying their behavior accordingly."
Loon pairs may live together in the same territory for as long as 20 years, which is half or more of the birds' lifetimes, Walcott says.
But the male and female remain independent and often look for new territories and mates.
The changes Walcott's team found took place when males flew around looking for new digs.
Lindsay said that previous work with loons hasn't looked at the effect of territory change on the birds' calls.
"People would say anecdotally that they could tell the difference between loon calls from different lakes," he said, "and that sort of anecdotal report made researchers want to look at this."
No one knows why loons change their tunes after moving.
Walcott first thought that females might prefer a certain call and that newly arrived males changed their yodels to accommodate them.
But males sometimes stay in the same territory and acquire a new female. In such cases, the male's yodel stays the same.
Then the researchers hypothesized that a male may change his tune to try to sound like one of the locals.
Wrong again. The changed calls were not imitations of the previous residentthey were always different.
Not all experts are convinced that a change in vocalization has any survival value or evolutionary significance.
"If this gave an advantage, then one would expect to see all males changing calls annually to gain this benefit," said Martin Fowlie of the British Trust for Ornithology, a bird-research nonprofit based in Norfolk, England.
"More research and experimental manipulations may give some insight into the adaptive significance of this change."
While the full significance of the call of the male loon is still unknown, it's clear that it is full of meaning to other loons.
"[A male in a new territory] must know the previous resident's yodel," Walcott said.
"And that's a surprise, too. It almost seems they're planning their moves. They spend time flying around, listening to responses to their yodels and strategizing about what it makes sense to do."
Lindsay, of Northern Michigan University, sees this work as an important contribution to the study of loons.
"These are charismatic birds," he said, "and they're fitting indicators of the health of our natural environment. I hope this study will encourage further work on these complex, long-lived birds."
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