writing a song doesn't have too much to do with ownership.

once someone senses there is money to be made out of it, people who are paying for the recording and marketing, demand the right to make a version that will shift units.

Then when it is in the public arena - the public sing it anywhere, anyhow, with whatever changes to the words and tune they want. There are exceptions -if you're a millionaire like the bloke from Oasis, you can take out an injunction and stop The Smurfs from recording your song. But even Noel can't change a million pub singers butchering his 'possession', smart arse parodist who couldn't create anything as long as they had a hole in their arse writing 'alternative' funny words. Politicians, football crowds or male voice choirs, ladies barbershop quartets, or Quentin Tarantino putting it in a film about people being disemboweklled. Ten to one, Paul MacCartney never thought his songs would be used as the soundtrack for public executions in Africa - but they were.

You write your song and it totters out into the world. That's it. Gone! No longer yours mate!