Pygmy Nuthatches (above) are one of the most social nuthatches in the world, and most times you won't just see one, you'll probably see 3-10 together. They share roost cavities and also have cooperative breeding. Basically, they form little family groups, with the young from past years helping with stuff like feeding, warning against predators, making cookies, doing the laundry, vacuuming. You know, generally stuff around the roost or nest site. (Other birds that do this are Florida Scrubjays and Acorn Woodpeckers)
(White-breasted Nuthatch above)
White-breasted Nuthatches don't do this and are more solitary. I think they're the most wide spread nuthatch in North America and have a bigger variety of habitat that they use.
(White-breasted above, feeding either female or nestlings)
Both White-breasted and Pygmies are secondary cavity excavators. They'll use old cavities (like an old woodpecker's) that they find and will use it as is or clean it up, make it bigger, etc. (They will also excavate their own cavity).
(Pygmy Nuthatch above)
I don't remember seeing Pygmy Nuthatches in a while and we came across about 4 of them sharing a cavity the other day. They're so tiny it's hard to tell who if any were fledgies. After closer looks, it doesn't look like there were any young, all adults. And all crazy cute.