Newborns. Eyes are closed, fuzz instead of feathers, looks like a bug and is the size of a shelled peanut, 1/2-3/4 inch.
Mother sits on them constantly, except to find food, but returns and keeps them covered and warm. She will sit on them until they are 7-10 days old. They are diligent mothers and tolerate rain, wind, and nosy humans.
If you find one on the ground and it is warm to the touch and moving, look for the nest and return it. Watch at a distance for the return of the mother. If she has not returned in 30-45 minutes, call a rehabber. What could have happened to her? She may have been caught by a cat, or hit a window, or gotten hit by a car, or was scared off by humans who keep taking pictures of the nest, or gotten caught in a garage. They do not usually willingly abandon their nests.
If baby is on the ground and cold, he must be warmed prior to returning to the nest. 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), return him to the nest 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. (see picture) 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 plain white 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 cold sticky water.
Feed baby every 30 minutes. Watch the side of his neck, if its bulging, then quit feeding until the bulge goes down. Keep reheating the sock 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 this is only a temporary diet until you can get him to a rehabber, who has the proper diet. DO NOT FEED HIM ANYTHING ELSE! There are crazy websites out there that tell you to give them cat food, or egg yolk, or seed based food ("Exact"), all of these are not good for him and can and do cause harm.
Recap - identify there's a problem, warm and watch for mom. If no mom, or too cold to re-nest, warm him in a home made incubator, feed small amounts of sugar water after he is warm and find a rehabber.