Some differences in the approach a songwriter might take to writing are:
- Priority of structure and rhyme - if your objective is a song, you're more likely to contain your lines and force rhymes into these lines. True, many poets do this in poems as well, but your choice of rhythms is somewhat limited by what is "singable".
- Necessity of a refrain - Folks songs excepted, most songs have a repeated section, introduced thematically early in the piece and permitted to return - this limits the length of content that can be presented. Mandates a stanza length, if you will.
- Constraint on length - a song, to function as a song, has a meaningful lower length limit and (great writers and bands excepted) a practical upper limit.
- More subtly, melody influences word choice, because some phrases become impossible (or unattractive) with some choices in musical phrasing. Of course, this works in reverse as well.
I've written three songs in my life that I'd be comfortable dusting off for performance, and none of their lyrics work as poems for me. Actually, one derives from a poem written by a friend in college, and in turning it into a song, I gave up quite a bit of his structure, to the point where he didn't like what I'd done with it because the structure loss dragged some meaning with it. Not good or bad, but not the same.
This is an interesting observation for me. I've been a musical performer (singer and instrumentalist) for over 30 years, a practicing poet for much less than that), which I think is why I naturally impose rhythm and sound (though not rhyme) at the expense of word choice sometimes. I guess I never really thought that through before. My friend, though also a talented performer, is by profession a teaching scientist -a purveyor of natural truth through precise language - and therefore less likely, I believe, to trust the music over the words.
This trade-off I see between music and meaning makes Ed Romond's accomplishment so much more impressive for me - the lyric functions as a poem and doesn't (appear to) sacrifice much in word choice for the sake of the song, but the song has a great rhythm and a memorable, hummable melody. And Ed can play guitar.