Have you figured out where your baby birds are? Listen carefully for tiny peeps and chirps. Usually, you'll hear more than one baby chirping at the same time, especially when food is delivered. "Feed me, Feed me!".
Cardinals nest in thick bushes or low in a bushy tree. Look very carefully, because when they build their nest, it blends so well with the branches that you almost can't see it. Look at it and then move away and please don't touch it. I'm sure the birds are watching you and their nest. If they feel you're a threat, they'll move on and build someplace else. I look for the nests, but I play like I'm looking at something else and then I move on. Believe me, they are watching!
Would love to hear about your discoveries!