It looks old enough to survive. Maybe tilt a cardobard box against the wall to give it some shelter and keep up the water and if you have any a small amount of seeds. I am willing to bet the mother will be back soon. I have seen ones this young have the mother bird come back to bring it with them. Until then, its just trying to keep them hydrated and a little shelter to keep them from being too cold/hot. If its really hot out, toss a water bottle in the freezer for a few hours then set it near by so it can get close to it if it needs to cool off. Not sure if that is the right thing to do. I have done something like that before and it seemed to help.