Guus Bosman

I never knew that the song 'Happy Birthday to You' is protected by copyright. This story however, describes the history of the song and the copyright transfers.

For private use you don't have to pay royalties but for commercial use you do. The song generates about $2 million in royalties annually.

From the article:
The next time you hear "Happy Birthday" in a movie -- and now that you’re listening, it won’t be long -- stay for the credits at the end of the movie.