If you find [what you think is] abandoned baby animals somewhere, leave it alone. Unless they're in traffic or something equally dangerous, leave it right where you found it.
Leave it alone.
Did I mention to leave it the fuck alone?
Yes, I know baby animals are adorable and look like they're about to die. It's a survival technique. They do that so that they get taken care of. This is also why human babies are fat, have heads that are proportionally way too big for their bodies and eyes that are way too big for their faces.
Mother animals transport their brood in small steps sometimes and can leave some of their young unattended at some point in time. But they have a system and they know what they're doing. And if they don't know what they're doing, if they're too stupid to know what they're doing, then they really should die.
If you remove the baby, the mother will come back, find the babies missing and move on.
In fact, don't even pick it up and then put it back.
Even if you put it back, there is a risk that the mother will come back and not recognize the babies as their own.
They will smell different now that you've put your stinky paws all over it. Then the babies are really screwed.
*sigh* RIP two baby birds who actually might have lived. I sincerely apologize on behalf of MY RETARDED SPECIES OF GIANT SMELLY LUMBERING MORONS. Gaaaaaaah.....
[Update - Sunday, July 29, 2007: 1200h]
I stand corrected. The parents wouldn't abandon their nest because of the smell of humans on their offspring. They could abandon their nest because of signs of disturbance.