Thanks to ChatGPT (which doesn't know enough about Opusmodus to be of much help, but has enough understanding of Common Lisp to be useful), I have a partial solution:
(setf flute-omn (gen-loop 7 (rnd-pick`(,bird1 ,bird2 ,bird3 ,rest1 ,rest2 ,rest3))))
It's a simple syntax problem. I needed to use the backquote (`) along with the comma (,) operator for list splicing. This still isn't doing what I want, per se, but I can figure out the rest from here.