The problem about caring for a wild baby bird, is you can do everything right, and things can still go wrong.
It is possible the baby bird was already compromised when it was found (injury, infection, deformity, undernourished by parents)
It is also possible that it may have had internal issues I couldn’t see - like neurological damage or organ defects. The height, from the bird nest and the porch the baby bird had landed on, was significant, and we couldn't reach it (else, we would have put it back).
Or it simply could be declining from stress, which is incredibly common in wild hatchlings without a mother’s round-the-clock care.
I've been caring for the baby bird every 20 minutes, but sometimes that's still not enough. My 2-hour mistaken nap, was likely not helping anything either, especially since the bird's eating was already on the decline (before my nap).
Regardless, I've provided warmth, food, hydration, and attention - things that have saved other baby birds in the past. Sometimes, though, even everything right isn't enough, and nature takes its course.
It is 8:30 PM now, and I've managed to get some food in the bird, before calling it a night. In the wild, the bird would be sleeping already, but I wanted to be sure they had that last, extra meal given the circumstances.
I hope tomorrow morning, I will hear a talkative bird, wanting more food.