Options, You can take it to a vet. You can try to doctor it yourself with a splint and selfsticking bandage wrap. You can leave it alone and let it heal, it wont heal properly but the bird will heal. Or you can put the bird down. Depending on your situation and what you keep birds for those are the options. I have taken chickens to he vet with positive results as well as negative results, with a broken leg I would try to doctor it myself because of the fact that whatever they do at a vet you will need to repeat as the bandages will likely need to be reapplied almost daily with regular activity. The best advise I have is to splint it and then put this bird in its own cage to minimize movements and further injury, keep a close eye on the leg and keep the splint intact as best you can but the less movement in the break the better the success of healing, after a week or so you should see if it is healing properly or not and then go from there. Either continue what is working or take another approach. It is very difficult to heal a broken leg on a chick simply because they will continue to try to use it and not let it heal properly. But on the brighter side if you do your part the bird can live with a crooked leg he will just be slower than the others.