We raised that house finch we rescued and it's a wonder she survived with us filling her up till her crop was bulged out all over but she did and actually thrived. We'd never fed a baby bird before buy she loved her formula and luckily would sleep all night once topped up. If your somewhat carefull about stuffing it in them and don't overheat it you'll probably do fine. We now keep some of the powdered baby food in the fridge in case another baby has lost their parents and needs help.
I spooked a baby ground dweller out of his nest I destroyed when cleaning up my back yard many years ago and mama was doing her best look at me I have a broken wing and would be easy to catch routine behind me. I finally caught the baby, topped him off with formula, and put him in the strawberry barrel we had in the yard, making sure to show mama as I did. Got up the next day and there was baby poop all over and evidence he'd spent the night snoozing and getting fed by mama but he was gone. I later heard him peeping all around our yard all summer and the parents looked after him and he stayed hidden till he grew up.