I thought it would be a good idea to make a Caresheet for people who find Injured birds, and have no idea how to care for them, so I am going to help, as I have been in the situation myself
The main problem with injured birds, is that they are unable to fly, remember this is a wild bird and may be protective if you try and pick it up, so its advicable to wear some thick gardening gloves when handleing
The bird should be placed in a shoebox or other contrainer, that has lots of padding and protection, you may use toilet roll, remember to leave the lid off, and leave it where it has room to fly out when it is ready.
IF you have the bird long enough to need to feed it, it is recommended to offer it earthworms, or seeds/nuts, as this is the food they eat out in the wild, you may also want to spray the nuts so they can get moisture.
I hope I have shown the basics of careing for a sick or injured bird, the main thing is to keep it warm, fed and to give it space to be free when it is ready.
could i just mention if you dont mind. that the information above relates to garden birds not birds of prey or any larger species. The best option if you think something is injured would be to take it to your local vet or wildlife rescue. most likely if i bird is injured it will be suffering from stress and even though you may think you are doing the best by feeding it, your not, your just making things worse.
You really need to know the age and (more importantly) the species of bird you have before you go feeding it. With very young chicks this can be quite difficult. Contact an expert.
as much as i admire peoples desire to help, the amount of birds that are unnecessarily brought to me in shoe boxes is ridiculous! most birds are fine where they are. if they're young, their parents more that likely know where they are (usually just fell out a nest) and will pick them back up. if they're stressed, shoving them in a box and trying to cram things down their throats really isn't helping. if its injured, it will either heal itself or die. no vet can save them, and treatments = stress. being stuck in a box = stress. being manhandled = stress.
if your worried about a bird, before even considering touching it call the rspb! they will advise you what to do.
agree with above statement, though if it's really cold and i find a bird that's flown into a window i put it in a box for a wee while till it's recovered (doesn't take long at all) then out it goes