Well? Did you bring it in or leave it?
If you brought it in then you can feed it with finch/canary/budgie seed or shelled sunflower seeds/wild bird mix chopped up small. Put in a small dish of water too. It'll be fine overnight and then you can pop it back out the front again to see if mum and dad come back for it
Sounds like it's been abandoned to me though...
If it's eating and drinking ok when in your care, then it will be fine on it's own in the big wide world
