Did it looklike this (the bird on the right)? If so it could be a baby cowbird. The female cowbird lays eggs in another bird's nest.

Hmm I've seen baby cardinals before ,and they have the full crest, but are slightly smaller than their mother.

Fledgling Cardinals are the same color as the female except with a dark beak. Crest can be put up or down so often you don't see it. Also fledglings birds by the time we see them following the adults around are about the same size as the adults. I would guess you saw a baby Cardinal being fed by its mom.

Female baby cardinals don't usually have the crest as fledglings. The males do. The females can easily be mistake for a sparrow as young fledglings (beak and feather pattern can help you tell difference) while the males begin to show orange and red hues pretty early. I've been feeding cardinal nestlings and parents for years now, they bring so much joy and will after time communicate with you. For example, mine are obsessed with blueberries. If I'm out of blueberries and don't put anything out, dad will pretty much "yell" at me now, or if I try to give them grapes or other berries instead he's even more annoyed LOL. And no, I'm not a crazy old bird lady. But birds understand more language than cats. My females are far more skittish, but my daddy cardinal has been with me 3 solid years now and will sit within a foot of me, let me move slowly, and allow me to talk to him. Mama, not so much. ;)

As for size, cardinals fledge quite quickly (11 days, incubate for only 13), but remain fully dependent on parents for at least a full 20 days when their flying begins improving. They don't venture far from their parents for weeks afterwards. But by the time they're flying well, they're barely smaller than mom. You likely indeed saw mom feeding baby cardinal. A baby cowbird doesn't resemble a cardinal, though cardinal parents adopt, incubate, feed and fledge them as their own.

My favorite shot of my Daddy Cardinal, Mr. Red, from this summer. That's a blueberry in his beak.

I'm amazed that you get birds living, what looks like, in an apartment, and cardinals yet. However, we don't get cardinals here (Saskatchewan, Canada), so don't really know much about their habits but I live in a small urban city and my back yard is very immature as we have only been here 2 years so no trees to speak of and I'm always hoping that some birds would find my yard (besides house sparrows lol). Wonderful to see that you can enjoy the beauty of the cardinals. I wish we would get them here but they don't usually come this far west.

Leigh, the picture you posted of a male cardinal feeding a female cardinal fledgling is actually a male cardinal feeding a brown-headed cowbird fledgling. Cardinals do not show any streaking on the body or wing bars. Nice photos though! I really miss cardinals since I moved out west.

Also, male cardinals will also feed their mates as a bonding ritual. It's really sweet to see when they do this. If the cardinal being fed was very young (juvenile) but not a new fledgling... that's not unusual because they are often fed by the adults for a good while after fledging. Also, the easiest way to spot a juvenile is by the color of the beak, which is usually all black or black with some other color starting to come in. Juveniles are usually the size of the adults.