To sing with a gentle, trembling quality; to produce a series of musical notes that rise and fall in pitch.
From Old French 'werbler,' possibly from Frankish 'werren' (to turn or wind). Connected to the idea of turning or winding around notes, creating a wavering sound.
Birds don't technically 'sing'—ornithologists call what they do 'warbling' because birds produce multiple notes in sequence, and some birds like the wood thrush can actually produce two different pitches at the same time using two voice boxes!
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