Birds sing four-part harmonies
“It’s already known that some birds duet and that others sing in choruses, but these wrens do both and, furthermore, the choruses are extraordinarily precise and well coordinated.”
Up to seven birds sing “choruses”, with males and females contributing different parts. The song consists of a series of four repeated phrases that follow the pattern ABCDABCD. Males sing A and C, while females sing B and D. Both males and females hit their notes right on cue so that the ABCD phrasing flows along as though only one bird were singing.
(It does sound like just one bird so I’ll just have to take their word for it!)