This was actually my first greater roadrunner I’ve ever seen! I turned the corner at this bird sanctuary outside Las Vegas, and there she was! Someone working there I showed this to suggested that it was sunning itself? I thought maybe it was surprised to see me since I was also surprised to see it!

this is a great pic! roadrunners and iirc some other birds will sun themselves this way, with feathers fluffed up so the sun can get closer in to the skin. it's thought to help get rid of parasites I think. a neat behaviour to see!
Fluff-butt!!
It's possible you interrupted a dust bath? Sometimes they scuffle around in the dirt, it helps keep their feathers clean and removes mites! Afterwards, they usually fluff up and preen and/or sun themselves
r/borbs
Ah! Yeah I should have thought of that!
Yes, that is definitively a butt. I'm so here for it.