What type of birds are these at my bird feeder?

I installed a bird feeder about a week ago with a general mix of seed (millet, sunflower seed, wheat, etc) and I've seen a few cardinals here and there, but most often I see one type of bird. They're very small; 4-5 inches in length maybe, and they have white bottoms and gray/black tops though the colors are mixed. Some of them (I'm assuming the males) have red at the top of their hears while the others have gray/white tops. I don't think it's any sort of special bird, but I often see upwards of 8-10 at and around the feeder so I'm just curious as to what kind of birds they are.