Patry Copyright Blog points us to this 67 1/2 page treatise on The Happy Birthday Song:
“‘Happy Birthday to You’ is the best-known and most frequently sung song in the world. …However, it is almost certainly no longer under copyright, due to a lack of evidence about who wrote the words; defective copyright notice; and a failure to file a proper renewal application.”
“It also reveals collective action barriers to mounting challenges to copyright validity: the song generates an estimated $2 million per year, and yet no one has ever sought adjudication of the validity of its copyright.”
Read all of it (here).