Nice shots.
They do that all the time. A friend and I call that the seventh inning stretch. Simply working the creaks out. If it were cooling down it would open both wings like a flasher in a trench coat. Seriously. Don't know how else to describe it. Lol. It's called Gular Fluttering.
@Joel Bolden made me aware of the proper term here:
Gular Fluttering
Have a ton of pictures of both. Recently got pics of one stretching on top of a wood duck house. It wasn't so much that it was stretching but, it did this ballerina act on top of such a small angled platform. And....didn't fall in the lake.