The moment a song is rendered in a tangible format, it is copyrighted. For example, whistling a melody doesn't assert copyright, but as soon as you write down the score or record the song to tape, it is copyrighted. Proving copyright is a different thing, however. To definitively prove copyright ownership, songwriters typically register the copyright with the Copyright Office at the Library of Congress. This creates a record of the song's existence as well as the owner of the copyright.
In cases in which two or more parties write a song, the ownership is split equally among all parties. The words in a song represent 50 percent of the intellectual property, and the music represents the other 50 percent. This gets complicated when musicians collaborate. If one person writes the lyrics and collaborates with another person on the music, the lyricist is entitled to 50 percent plus at least half of the other 50 percent, depending on their contribution. Typically ownership rights are covered in a band's contract, and the goodwill among members plays a part in the arrangement.
Mechanical rights concern the sound recording of a song, as distinct from the intellectual property of the song itself. For example, if you release a song, you receive royalties for record sales and separate royalties for performance and use of the song in other forms, such as cover versions, reproduction of the lyrics and soundtracks.
If a third party wishes to use part of your song in their own recording, they must seek copyright clearance from you. Failure to do so puts the third party at risk of copyright infringement. British singer Robbie Williams was forced to pay damages to Ludlow Music after a court found that lyrics in Williams' song "Jesus in a Camper Van" were too similar to lyrics in the song "I'm The Way," by Loudon Wainright III. Any specific sounds from your recordings are also protected by clearance. If a third party wishes to sample any part of a recording that you own, you have the option to grant or deny copyright clearance. Typically copyright owners grant clearance in return for a royalty.
Copyright protection does not extend to cover versions. It's not necessary for someone to seek your permission to perform a cover version of one of your songs. Once a song has been published, it is in the public domain and can be re-recorded. You are still eligible to receive 100 percent of the performance rights for the song, however.
The term of copyright protection lasts for the lifetime of the songwriter, and extends to 75 years after their death. Following this period the song becomes a public domain work, and anybody is free to use it.