Not many people are aware that the song "Happy Birthday To You" is not in the public domain but actually owned by a music publishing company. That is why if you appear on TV to play an instrument, you are told not to play Happy Birthday, since the TV station doesn't want to pay a royalty on the song.
The rights to the song is owned by a company called Warner/Chappell Music Inc. which owns a huge library of songs including the songs of Cole Porter, George Gershwin, Dr. Dre, Eric Clapton, Led Zeppelin, Madonna, and Sheryl Crow. Warner/Chappell is in turn owned by Warner Music Group Inc. (WMG). Warner Music collected about $5000 per day, or over $1.8 million per year in royalties for the song, and they are very strict about collecting their royalties. The song was first copyrighted in 1935 and won't go into the public domain until 2030.