Copyright usually is more complicated when it involves songs. However there is no doubt that you still retain the copyright in the original composition. The adding of lyrics merely turns a piece of music into a song, and because the music and lyrics are distinct from each other, they can be treated separately for the purposes of credits and royalties for the song. Take a look at another recent thread
here in which split sheets were discussed. This should provide a little more detail about that aspect.
The addition of percussion to the original composition may or may not be significant, depending on how much it contributed to the 'shape' of the song overall. As a musician you can probably decide that, and if so, decide how much extra it contributed. But even with the addition of, say, a strong beat which wasn't present in your composition, your underlying composition still remains the heart of the music part of the song. The drum part would need to be 'original' in the sense that it was not just a bit of tempo and a few cymbal crashes, which might be found in any song of the same genre. A standard ska beat for instance would not be 'original' in this context.
Let's assume that the drum element is significant to the music. In effect there are three different copyright works here: your original composition (let's call it the melody), the music is the melody with the drum element added, and is known as an adaptation
, and the music+lyrics is the song.
I assume that you are asking because there may now be some dispute about who owns what and what can happen to the song as a whole now that the band has split up. The best way to approach this is to agree that the song (music and lyrics combined) is co-owned by you and the lyricist (ie the singer), and if the drum part is significant, the music is joint-owned by you and, presumably, the drummer.
Co-ownership is different to joint-ownership when it comes to dealing with a copyright work. Joint ownership exists where the individual contributions cannot easily be separated afterwards (like taking the flour out of a cake). So where joint ownership exists, both creators must agree to any subsequent exploitation of the work which has been jointly created. However since the lyrics are entirely separate from the music, copyright ownership can exist separately, and in this case it will belong to the singer who wrote them. A co-owner (A) does not need the permission of the other owner (B) in order to exploit A's work on its own.
The percentage of each of the shares is hard to give general rules about, but the video about how split sheets work which I linked to in the other thread can help to explain one approach. If none of you can agree to a split, then in theory each contributor can refuse to allow his/her share to be used by any of the others. Since your part, the main melody, is fundamental, you hold a strong card in any negotiations. For example, new lyrics could be set to your melody together with a different beat thus creating a completely new song, to which none of your other former band members would be entitled to any share.
If you have a little time to spare over the bank holiday, you might like to look at this recent court case (Minder Music v Sharples
) which involved many of the elements I have discussed here. If you are short of time, skip down to paragraph 66, as the earlier part of the judgment is largely about whether certain agreements existed between the parties.