I found an injured bird what should I do?

Best to take it to the local vet and see what they can do for it, i can see what you mean by they may put it down but if it can never fly again then it will be hard for it to survive because it will not be able to provide its self with food, if the vet dose say this depending on what bird it is you may be able to keep it as a pet. best of luck, so nice of you to try and save it :)