A Northern Cardinal's diet mainly consists of seeds and fruit. They readily eat grasses, sedges, buckwheat, sumac, mulberry, blackberry, hackberry, and grapes. In my experience, they will surely eat corn and they seem to love dogwood seeds. They will respond to many types of seeds but black oil sunflower seeds must definitely be their favorite seed. Their insect diet (and this is mostly what the fledglings receive) crickets, leafhoppers, cicadas, moths, centipedes, spiders, flies, and even butterflies. Yumm.