I am reasonably sure my unidentified visitor is a Brown-headed Nuthatch. Here are a couple of better pictures. Check this link and see if you think they are the same thing.
This guy is a little acrobat! He will tilt forward and swing all the way around until he's upright again. Sometimes he just likes to hang upside down to eat. The link says "The Brown-headed Nuthatch is one of only a few bird species in the world known to use tools. By holding a small piece of bark in its bill, they pry open bark on trees to expose insects." They are also fond of sunflower seeds. (I could have told you that!) Hop over to the link and see what a group of Brown-headed Nuthatches is called.