As I noted in a recent post about the dark-eyed junco, juncos most often eat on the ground and aren't commonly seen at our hanging feeders. Last weekend I happened to capture this junco's visit to a snow-covered fruit-and-seed block. At this angle the bird appears to be straddling that cage and the nearby wreath-style feeder we use for peanuts in the shell. A sunflower seed is in its beak.
The lighting's not great on this photo of a white-breasted nuthatch, but I liked the face-on position and the half-peanut in its beak.
Here's a male cardinal with a sunflower seed.
It's interesting to watch the different ways birds get seeds and nuts out of their shells. Birds with heavier beaks that are suited to the task, like cardinals and finches, will hold the seed in their beak, manipulate it with the tongue to get it in just the right position to crack the shell between their upper and lower mandible, and then eject the shell halves while retaining the seed. Other birds, like the black-capped chickadee, will hold the seed between their feet and crack the shell by pecking at it until they can get the seed out. And nuthatches get their name from their habit of tucking a nut into a crevice in the bark of a tree and then hacking, or "hatching," it out.
I found a kid-friendly website that does a nice job of showing the various beak adaptations and corresponding eating styles, as well as a lot of other information about bird physiology and behavior: Project Beak (the link shows the "cracking" beak of cardinals, sparrows, grosbeaks and finches; there is a dropdown menu from Beaks on the side menu to see other beak styles). They also have a nifty Build a Bird feature, where you can select different styles of wings, heads, feet and habitats to put together a proper bird or one of your own invention. Here's mine: