Yesterday in my back yard I noticed frass on my coneflower leaves. I looked around and saw that the old flower stems I'd snapped in half a few weeks ago had been hollowed out by mason bees. You can see a little bee butt sticking out a little in one of the pictures. So, note to self: Leave the old stems for the little bees.
(The top photo is a freshly snapped stem that is still solid with pith).