I have spent HOURS taking care of baby birds that people have found. Some make it, some don't. Sometimes there is an unseen birth defect that the mama bird can sense and therefore will sacrifice the baby (push it out) so it doesn't consume food that could be eaten by the healthier babies. Sometime they just get accidentally pushed out.
My advice is this: DO NOT PUT THE BABY IN WITH YOUR CHICKENS FOR BIOSECURITY REASONS. That baby might infect your flock and it could be a disaster.
Then, if the baby is still alive as of today, start by getting a medium to small bird cage and putting paper towel on the bottom for the first week. You need to monitor it's poop. Not just what it looks like but also "how" the baby does it. Also have an eyedropper and try giving it just water or if you have poultry electrolytes that is better, or just water down some Pedialyte. These products should only be used for max 12 hours just til you can get to the store. What would be better yet, and you can use for a little longer if you have it is Gro-gel Plus (more watery than the directions just for ease of squirting it out of the eyedropper) and drop a little at a time just inside the birds beak. DO NOT STICK THE DROPPER FAR INTO IT'S BEAK. If it is healthy it will suck it down itself.
There are so many precautions and other details on doing this but the most important thing is give a little at a time and don't force it.
As Soon As Possible get some "Exact" hand feeding formula for all baby birds by Kaytee. I believe most large pet stores carry it.
Whatever you have started with, water, electrolytes, Pedialyte, or Gro-gel, be sure that you start the first half dozen or more uses of Kaytee by mixing this with it. The baby should "squat back" and then poo every time after it is fed, you can always offer more food after this, but generally they may not take it. Some are pigs and do though. Watch the crop. Don't feed the baby if the crop is still full. No exact timing on feeding, when they are fuzzy it can be every hour but sometimes they can go for a few hours and still be fine.
The age of the bird is key. If it has all fuzz it will have to be watery, some feathers and fuzz you will have to make this a little thicker, no fuzz at all then more thick. If it survives past a week and you start to see tail feathers, add more protein. First I start with tropical fish food flakes mixed in with the Exact and then after a week or two graduate to frozen brine shrimp again - MIXED INTO THE EXACT and administered with the eyedropper. Plastic eyedroppers are best because there is no chance of breakage and if the food gets too thick you can cut the tapered end off.
If you get past this point, 2 -3 weeks, email or pm me and I will be happy to help you through the process until release.
If you do fine on your own, a word of caution, if they start practicing to fly in your house but are not ready for release, COVER WINDOWS, MIRRORS OR ANYTHING LARGE AND REFLECTIVE AND PLEASE
TURN OFF ANY CEILING FANS!!!
Wild baby birds are so much more work and more involved than baby chicks!!!!
And PLEASE don't handle the baby and then go handle the chickens or vice-versa, without washing hands and changing clothes, and don't keep them in the same area. If baby is in the house, and you have the air conditioner on, move the cage to the warmest room in your house and keep the cage covered when not feeding it. Don't over-handle him and check for drafts. (Light a candle or match, move it very very slowly around the cage and see if the flame flickers at any point.)
Please be aware though there is no "for sure" in this. I AM NO EXPERT, I JUST DO IT. I have had babies die in the first 12, 24, 72 hours and even after 2 weeks and another 1 day before release and with the exception of one that had no tail feathers and had been mauled by a cat, I felt they all were doing well and thriving just before they died.
I have had several fly off into their rightful lives as wild birds too. I only started doing this because my kids would find them and bring them home and no one I contacted could or would help. If you are successful then for, well eternity, you will have people bring you the baby birds they find and then THEY will be crushed it it doesn't make it.
Now I have gone on way too long and it is time to feed the baby Robin my son found yesterday.
I actually began this "rescue" adventure with abandoned kittens. One year it was a baby deer, but that is for another forum . . . .