Researchers studying birds called Japanese Tits, relatives of the chickadees and titmice in North America, noticed that mates raising chicks together often fluttered their wings near the entrance of their nest box. After recording hundreds of examples of this behavior, it became clear that the wing-flutter was a signal for the other bird to enter first, much like that arm-sweeping gesture that people use to mean, “after you.”

More info and transcript at BirdNote.org. 

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