Like chickadees, they take individual sunflower seeds and hammer them open to eat the good part.
They also have a distinctive call which is easy to recognize - a nasal 'hank, hank'.. During summer you're more likely to hear them than see them.
The Red-breasted Nuthatch is a bit smaller, with a very distinctive eye stripe and rusty brown colour underneath. We have seen them much less frequently, as they inhabit coniferous tree stands. We do have numerous blocks of conifers within sight though, so maybe that's why they are visiting.