About 21-24 days old. Length is 1 1/4 inch - 1 3/4 inch. It intentionally leaves the nest to start it's "training", learning to fly, feed and to be a bird. It is fully feathered, but it's beak and tail are shorter than the adult's. It is unwise and does not know to be afraid of cats, dogs, people, cars, etc. It is easily caught. Will often land on chairs, grass, driveway, car, pool. Remember, it is only 3 weeks old and has no idea what these things are. It cannot take care of itself, or feed itself yet. It is completely dependent on the mother.
Think about a human baby. He wants out of his crib, he can walk a little, but still needs mom for protection and food. She feeds him when he cries. He has no life experience and is extremely vulnerable. This describes a human fledgling. Bird fledglings are the same. They can fly, but have no common sense. They cry when they are hungry and then fly off into trouble. It is a wonder any of them survive.
If you find one on cement and it is alert, put it in a nearby tree or shrub. If it is in the middle of the street, or on your car or it lands on your head, place it nearby on shrubs or in a tree for the mother to find it. It will peep when it is hungry and call her. Do not move to a different location. Other birds will not feed it and it will die because it does not know how to find food. It must be near it's mother. (Dad doesn't feed the kids, he is just the baby-daddy). Watch at a distance for the return of the mother. If you found it in your yard, keep it safe by keeping cats indoors and limiting the dog's potty-time. Baby will be gone in about 2 weeks, so enjoy the magic of watching a baby bird turn into a teenage bird.
If it is crying and is puffy and has it's eyes closed, it needs help. If it's laying on its side or bleeding, or has been in a cat's mouth, it needs help. Cats have bacteria in their mouths that are toxic to birds, so this bird needs to be on antibiotics. If its been in the pool and is cold and wet, it needs help. If it has been sitting in a soccer field, with a game going on and the mom can't get to it to feed it, it needs help.
If it has bugs (mites or ants), it needs to be rescued.
If baby is on the ground and cold, he must be warmed. This can be done by holding him in your warm hands for 10 minutes. If he gets active and gapes (opens his mouth to be fed) or cries, give him a little sugar water and return him to the area where he was found (if it's safe) and observe for the mother to return. If the mother has not returned in 30-45 minutes, call a rehabber. If he does not become active and gaping, call a rehabber. Until one can be reached, the baby needs to be warmed and kept warm. So this is what you do.....
Find an athletic sock and put in 1 cup of uncooked rice. Put the rice sock in the microwave for 1 minute. Cover the warm rice sock with a towel and place it in a shoebox lined with a towel or tee shirt. Use a salsa cup, or a bump from an egg carton, put Kleenex in it and put the baby in the cup. Put the cup in the shoebox with the warm rice sock (close but not too close) and cover the box with a lid with holes. DO NOT FEED THE BABY YET! Baby must be toasty warm before feeding is attempted, which will be at least 30 minutes, if not longer.
Once baby is warm and active, its ok to feed just a little. Mix 1/4 tsp sugar in 1 tsp warm (not hot) tap water. Dip a q-tip in the sugar water and put to baby's mouth, give just a little. If you have an eye dropper, you can give 1 small drop into the open mouth. If you miss, or some gets on the baby, take a damp q-tip and clean him up. Don't let him sit in sticky cold water. Feed baby every 30 minutes. Keep sock warm and don't let baby get cold.
He needs more than sugar water. Mom gives him a special blend of flower nectar, small flying bugs and sugar water. So the sugar water is only temporary until you can get him to a rehabber. Please do not feed him any of the weird things you find on the internet. They do not eat egg yolk (they came from an egg yolk, so yuck), or milk, or bread or cheese or meal worms. Rehabbers have the correct diet, so please get it to a rehabber. They are not a DIY project.
Recap - identify there's a problem, warm and watch for mom. If no mom, or too cold to re-unite with mom, warm him in a home made incubator, feed small amounts of sugar water after he is warm and find a rehabber. ALWAYS OPEN THE BOX IN A BATHROOM. IN CASE THEY FLY A LITTLE.