Minor notes:
- If you rephrase the original question slightly, it is: "If I wrote part of a song, did I write part of the song?" Well ... yeah.
- How big a "part" of the writing is sufficient to "hold copyright," as they say, is not an obvious or easy question. It has to do with fairly nebulous and not-consistently-thought-out considerations, like distinctiveness and originality. Suggesting that you add a drum roll between the end of the first chorus and the verse probably doesn't make you a co-writer. Giving a suggestion to a songwriter who needs a word to fill the line "a trip to the moon on ---- wings" presumably doesn't either, even if you are clever enough to suggest "gossamer" (anyway, the guy who did exactly that didn't try to collect royalties). On the other hand, I believe there is a case (also involving Cole Porter, though I may be remembering wrong) that held just a rhythmic figure could hold copyright if it were sufficiently distinctive.
- There's quite a bit of history, at least in the early days of rock music, of people being aggressive in claiming copyright based on very minor (if not entirely nonexistent) contributions. Typically, these involved producer-types trying to horn in on royalties as "co-writers" of songs that came from under-sophisticated performers. I think Frankie Lymon and the Teenagers won some litigation, though lots of others just gave up a share of their royalties. This pattern even formed the basis for a little plot-let on "The Sopranos" early in its run.
- If you're going to get into melodies versus chord patterns, your collaborator (as others have suggested) is utterly and compltely backwards. Words and melody are the basic fundamentals that clearly hold copyright. The hard question is whether contributing other things makes someone a co-author. If chord progressions were the whole thing, then every song with a I - IV - V or I - vi - IV - V progression would be in the public domain, which I think would knock out a huge part of the entire rock songbook.