I'm always surprised when people say they have a baby bird this late in the year. Except for pigeons I don't know of birds that breed this late.
In any case, the best thing to do is find someone who is licensed in wildlife rehab care. They have the necessary equipment (special feeding tools, incubators, proper food mixes for the various species) and they have a better chance of success with the little one.
Some babies are reluctant to open their mouths because you don't look or sound like "mom." You are an unknown predator and they will scrunch down and avoid you. Some babies need to have their mouths gently opened and fed that way. Others are so reluctant that they need tube feeding for a couple of days until they make the connection that we are the bearers of food. And some will only eat when put with their own kind.
Please look online for a rehab facility near you or check with some vets, the zoo, the Fish and Wildlife office in your area. Someone should know of a rehabber nearby.
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