Intense Nest Activity in New Boxes

Mr. Curiosity developed an itch today to investigate the current state of the two newest nest boxes. They're identical, mounted on posts, and set back a little in the woody area. And today, both had masses of thick twig material. Whodunnit? This is not the style of our Carolina Wrens which use a lot of pine needles. Nor does it match the known work of Bluebirds, Titmice, or Nuthatches. It's not as tidy a job as I'd expect of Robins, and not quite the Cardinal style, either. I've seen this pattern once before and given its feeder site at that time attributed it to a small bird. Very strange stuff. My nest ID skills are rather weak hence I am leaving myself baffled and hoping I can catch a glimpse at the perp in coming days. Ideally, I hope a hatchling comes of the thing(s). We shall see.