When Is A Musical Introduction Copyrightable? Katy Perry, White Zombie, The Pixies, Neil Young, Isaac Hayes

I thought I would start to listen more carefully and analyze introductions to songs to find the least and most COPYRIGHTABLE introductions, as well as the "in-between" introductions, i.e., introductions that would exhibit some copyrightable elements. I believe that there is a sliding scale of copyright protection - that some introductions are not copyrightable as musical compositions, some slightly so, some more so, and some extremely so. The length of the excerpt, both in terms of temporal length as well as number of attacks and/or articulations, as well as its originality would also be factors used in determining how copyright protection should be afforded an introduction.