we don't get a lot of robins and bluebirds that stay around in the winter but for those that do we put out raisens and dried cranberrys and suet and shredded or diced little pieces of leftover roast or pork chops. live mealworms or grubs freeze in munutes out in a dish. so some people get the dried mealworms and put some out.
the robins when they return in spring usually eat the left over berrys and fruit on the trees and shrubs but this winter the starlings have stripped the goodies from most bushes and trees. so for the robins we dice up bits of fresh fruits or berrys and put out a little at a time.

yes they are mostly ground feeders but in spring and summer they eat mostly bugs and worms and insects but in fall and winter they eat from bushes and trees and will go to feeders. The people that feed mealworms or grubs ect often have a glass bowl on a plarform feeder or hanging from a branch, so they put the berrys or dried fruits or meat scraps in the bowls or platforms which are often baffled to discourage squirrels. also some people put suet or winter foods in small containers like film canisters or other small tubes or bottles and either tape or hang them in bushes or from small limbs that the squirrels can't access easily. myself i have a plastic lid on a piace of wood hanging down from a small limb and put foods or suet or berrys ect on the lid. My worst problem in starlings that will eat anything right away so it's a battle sometimes to scare them away and put out small amounts as needed