How can you tell the sex of a wild bird

Question
Me and my friends at summer camp found a bird nest at the bottom of the tell next to the school. The mother bird was no where in sight so we decided to adopt the three little bird. They are very young and just today we were trying to figure out the sex of the bird. But we didn't know how to, plus we don't know what kind of species of bird it is. Please help me and my friends

Answer
The sex of young birds is very hard to tell.
Normally a bird vet can tell the sex of those birds and what kind of species it is, but even those people have a hard time to it.

For the sex they use a feather and some DNA analyzing tools or a very small look operation.

What kind of bird it is, that you can find in specialized books. You can find those in a public library.