The "Tremolo" is a Warning Loon Call

The tremolo is the familiar, loud call that sounds like human laughter. But sounds are deceiving, as the tremolo is chiefly a close-range alarm call that adult loons produce when they or their chicks are threatened by eagles, large aquatic vertebrates, or humans that venture too close to them.

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That’s a loon in the background, it’s yodeling. Wonderful creatures.