Leave it alone. It's most likely a fledgling and the parents are still around to take care of it. You may not even see them. As they get older the parents don't feed it as often but are still around.
It may be a fledgling (Baby bird) that cannot fly yet. Watch and see if the parents come ever to see if they feed it. Generally the parents will find bugs to feed the fledgling until it can fly. It may just jump out of the tree at some point and land on the floor, the parents should still come to feed while it is on the floor and still cannot fly it so keep an eye out. Eventually after flapping about and practicing to fly it will get stronger and fly.
You must keep very quiet and hidden as you may be scaring the parents off from feeding the bird if they see you, try to keep well out of sight but where you can see. If it is busy with lots of noise the parent may be scared away from feeding it.
A key thing is to keep cats well away from the fledgling incase it comes to the ground as they will chase it and kill it. Shut any cats indoors with a litter tray to go to the loo.
If no parents come to feed it at all over a very long time say a day, or it is an adult bird which is injured you may be able to get some NO KILL animal rescue groups to help, try looking in the phone book and phone around. Always ask if they have a no kill policy and some "shelters" kill the animals that are poorly or do not get rehomed quickly. They may just give you advice on how you can help it yourself.
Cook it up and eat it. Natural selection.
RSPCA
There's a bird outside my house on a small tree that can't fly. It's been on the same tree for over 24 hours. What do I do?! Who do I call?!
