Found these Carolina Wrens building a nest recently and watched them for a while. You can see a spider web mess hanging behind one of them in the vertical photo. One of them got that snagged while trying to get it into the cavity and left it there. A Black-crested Titmouse came by and tried to snag it and some chaos ensued. Kind of fun to watch, though no photos of that episode. I couldn't get any great nest material pictures because they'd always go head first into the cavity and butt shots are boring.