Hi Mary Ann,
I’m so sorry that this happened. You did the right thing by only observing and not interfering until it was clear the baby was in distress. Song bird chicks will fledge out of their nest before they can fly, but they can generally make it to low bushes or branches Some chicks get too active and fall out of the nest too early. When possible, we recommend placing the chick back in the nest. Unfortunately when they are going through this stage of learning to fly, they spend a lot of time on the ground and are vulnerable to house cats that are allowed to roam. This is the most common cause of an injury like you described. While cardinals can be very territorial, I doubt it attacked the chick, and any injuries would more likely be on the head or back. This little one may have been injured in the nest or in the fall from the nest. Momma bird did her best to take care of him but a wound like that would be fatal. At least the chick had you and his momma watching over him in his last hours.
With baby bird season upon us, this happens a lot and we are actually presenting a webinar on this subject this Friday, June 5th. It is free to view so please join us if you are interested.
Webinar: If You Care, Leave It There: Helping Baby Wild Birds
Take care,
Brenda