This is a very difficult question to answer without knowing what type of bird it is that was found. There is a website which has excellent instructions on how to look after orphaned baby birds. www.placeforwildbirds.com. (I have placed a period at the end of this URL to keep it from being an active link.) I am not associated with this site but have found it to be very helpful.
Judy from the-scoop-on-wild-birds-and-feeders.com