Grow some flowers that ground-feeding Northern Cardinals enjoy so that you attract more of them to your garden. Also, don't forget that they need cover and places to raise their babies so think about trees and shrubs that fruit such as dogwoods and serviceberries, and stock your feeders with black oil sunflower seeds.
Since cardinals don't migrate, try to provide a sequence of flowers and berries and seeds throughout all four seasons. Red fruit plants such as hawthorn, raspberry, strawberry, sumac, winterberry, and holly help male cardinals form their red feathers because red berries contain carotenoid pigments.
We also need evergreens for them to shelter in during the winter, low-to-the-ground boxwoods, for example, as cardinals build their nests fairly close to the ground. Plant dense small shrubs and trees such as arborvitae, spruce, and junipers. Also, caterpillar-host plants such as dill, fennel, hollyhock, and snapdragons attract butterfly and moth caterpillars and are great sources of food for cardinal parents to feed their babies.
And remember plant lots of nasturtiums, coneflowers, and sunflowers to provide seeds for these year-round garden residents. Leave the deadheads on for their winter nourishment.