Cardinals need pure safflower seeds. Different birds require different seeds. You may think that the birds in your yard are eating and enjoying the 
particular seed you put out because the feeders are always needing to be filled. You need to really watch your birds when they feed.Different birds require different seeds. You may notice that the birds are throwing the seeds onto the ground, not eating them. This is especially true if the see you buy has fillers in it. Birds are notorious for dumping out the fillers. Check the ground around your feeders and you may see seed sitting on the ground. This seed is just waiting to sprout and when it does it attracts unwanted animals like mice.
Safflower seeds are small and white; and are loaded with fat and protein. Besides Cardinals there are many other backyard birds that enjoy safflower seeds- including House Finches, Mourning Doves, and White-breasted Nuthatches. Safflower seeds are not too expensive and will last longer becasue the birds will actually be eating the seeds, not kicking them to the ground. A great plus with using the safflower seeds is that squirrels will not eat it because they find the safflower seed to bitter for them. In addition blackbirds and grackles usully do not like the taste of safflower seeds.