Thank you for your lovely comments.

leeneia, it's a matter of perception, but for me the altering of the onomatopoeic music with the "cuccu" words so that that the music is no longer onomatopoeic and therefore no longer mirrors the previous "cuccu" phrase is fundamental to the musical meaning of the song (I explain more about why in the article). Secular words in a manuscript isn't so surprising: there was a lot of medieval two-way traffic, secularising religious songs and Christianising secular songs.