In 2008 the Fifth Circuit Court of Appeals issued an opinion against copyright holders after failure to enforce trademarks for more than 40 years.
The defendant Thomas Kenneth Abraham for years was a producer of decorative fraternity and sorority paddles. In 1990 thirty-two fraternities and sororities commenced contacting Abraham asking his to pay a license to use certain house names and logos. The defendant asseted a "laches defense" which is when a copyright holder falls asleep which regard to their trademark enforcement as he had already been creating the paddles for 30+ years. In 2008 the plantiffs sued and and were declined monetary relief after failing to enforce their copyrights for so long. They were however awarded an injunction against further use by Abraham, which of course doesn't line the attorneys pockets.
How long have people been singing Happy Birthday? How many people even knew there was an "author"? I would have assumed it was a 'traditional' and hence public domain.
Certainly there has been no enforcement of copyright over the years, and I'm guessing latches would apply.