Sometimes they just fall out of the nest. If you can find/reach the nest, put him back in, or very close by on a high branch with some soft materials to protect him, so the mother can find him, and he'll be safe from predators. If she doesn't return in a little while, then take him to the center.
If you are not far please take it to the center as soon as possible, they should be able to help right away
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