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Black Sunflower seeds are a favorite of cardinals. However, cardinals also like safflower seeds and white milo. If you are concerned about litter from seeds, you can buy hulled sunflower seeds. Just make sure to change these seeds more often, as they don't have the same protection from going bad as whole ones with shells do. Cardinals like large seeds. They will eat smaller seeds if nothing else is available, but they prefer large seeds, such as sunflower, safflower, peanuts, or cracked corn. In the winter use a suet feeder to provide extra energy for cardinals and other birds. While this will probably not be the primary feeding choice of cardinals in your area, they may come to a suet feeder to find extra food in the winter. Suet should only be fed in the winter. This is because it is essentially a block of animal fat mixed with seeds. If it gets too hot, it can melt and go rancid. Cardinals are actually ground feeders, so they will happily eat off the ground. If you have a patch of ground that is away from low bushes or shrubs, where predators may hide, you can also scatter seed there. Be warned that ground feeding will also be attractive to squirrels. If you have a lot of squirrels in your area, you may want to avoid ground feeding and use squirrel baffles or squirrel proof feeders.
Put sunflower or safflower seeds in your feeder. Put a suet feeder out in winter. Put food on the ground as well.