How do you feed a baby bird?

I found this baby bird in the grate to my fireplace, the nest is in my chimmy so there is no way to get it back to the nest. I called animal rescue and they will be here in the morning but that is at least 7 hours away. It needs to be fed every 15 to 30 minutes. I have done everything it said to do to try to feed it. Is it simply not hungry or do I force its mouth open? It has been stuck there for about a day...so I would think it would be hungry.