Song Duels?
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Marsh wrens have large song repertoires, often over 100 themes, and tend to sing their way through their repertoire, one song after another, in a particular order. The order seems to be a matter of local consensus.

Because all of the males in an area tend to sing the same songs in the same order, one bird can generally anticipate the songs of another. This may be part of the origin of what's called matched countersinging, where one marsh wren will sing a theme, and another will immediately sing that same theme. The first bird sings a new theme, and that, too, is immediately repeated by the second bird.

Why do they countersing? Is it a competition, a duel, to see who's the better singer, presumably more attractive to female marsh wrens? Or is it maybe some less competitive social ritual? Or something else entirely?

Duelling or not, it's an intriguing display of matched singing.

Chris Tenney recorded these marsh wrens near Salinas. Joseph V. Brazie's recording of matched countersinging in marsh wrens can be found on the CD accompanying the Cornell Lab of Ornithology's Handbook of Bird Biology. The snippet from "Duelling Banjos" is from the Deliverance soundtrack CD featuring Eric Weissberg.