I believe that I should try the Preview option, which might help me catch typographical errors and improve my wording. At least I'm learning early on.

In my first post I should have said sung folk songs. It's true that very many folk songs are instrumental.

I've not done academic research on the question yet. I got to thinking about the question while trying to write songs as a not very skilled instrumentalist. Now I'm often conscious of the question as I listen to songs, particularly old ballads, songs that accompanied a particular activity, and so-called campfire songs.

Many so-called singer-songwriters of recent decades, I've noticed, rely on fills quite heavily; I can't entirely knock the practice, and I like many of the songs, but I find interesting the distinction between songs with and songs without fill spaces. In some cases held notes, such as whole notes, might allow enough time for fills, but that's not a given in the old songs.