This morning, when I went to get my clothes pins out of the tool shed, I noticed the nest remains on the ground. The branch it had been attached to was broken and pushed aside.
Probably been the work of a hungry animal in the night.
One little bird (it had some feathers and bald spots) was clinging to a Rose of Sharon branch but it seemed to be failing in the heat, its head down. I ran inside and googled a wildlife rescue site to find out what to do.
I put the nest remains in a strawberry basket, got on a ladder and wired the repaired "nest" to a branch near the old location, then approached the little bird to try to put it back in the new nest. But before I could do so, the parent birds flew near and the baby bird fluttered a few inches down to the ground and tried to hop away.
Following online instructions, I left the birds alone, since the parents were still trying to look after the baby. It hopped under our high fence into the yard next door, where I couldn't see it.
Though the parents chirped and fluttered around the fence for an hour, they're now gone.
I'm almost certain their last baby didn't survive, although I hoped it would have enough strength to somehow learn to fly.
I feel sad for the cardinals. I'd watched them work so hard to raise their little birds.