What to do for a baby bird alone in its nest if parents are gone?

His best chances will be to stay with his parents. If you can see where the nest is, you may briefly pick him up to place him back in the nest. Most birds do not actually have a very good sense of smell, so your scent won't cause a problem - however, your presence will frighten the parents away. If you cannot return him to the nest, the best course of action depends on how old he is. If he's got feathers and can hop, he's probably a fledgeling who is learning to leave the nest and you should leave him alone. If he's just starting to get his feathers and seems bright (can lift his head and look around), make sure he's in a safe spot (you can make a little nest on the ground with a clean towel in a small box - but don't move him far from where you found him, so his parents can find him there too) and see if the parents come to feed him. If you're convinced he's on his own, or if he's too little to have any hope of surviving on the ground (even if the parents stop by to feed him), then I recommend finding your closest wildlife rehab location and calling to see if they'll take him. Hope this helps!

If this is a wild bird the best option would be to get it to a wildlife rehabilitator asap. If a pet species you need to know what species in order to feed approriately. You need a hand rearing formula suited to that species. Keep in a "nest" with supplemental heat and humidity as it cannot regulate body temperature yet. Relative humidity greater than 50% is required initially. Hatchlings (no feathers) should be maintained at 95° - 97°F (35° - 36°C). An incubator is usually recommended for hatchlings. The yolk sac should provide nutrition the first 12 or so hours but but it will then need to be fed every 2 hours around the clock Here is some basic info https://vcahospitals.com/know-your-pet/hand-feeding-baby-birds

Hello, the little bird likely fell out of his nest. If it hasn't been too long, you can take it back to where you got him to see if his nest and mother is still there, which would be best case scenario. Otherwise, it may take him some time to learn to fly on his own and you'll have to wait until his flight feathers grow fully. In the interim, you may continue to feed him baby food, or you may start to offer seeds, fruits, and veggies. Hope this helps and best of luck.

As long as the parents are taking care of the hatchling themselves, you shouldn't need to do anything as they will keep it warm by laying with it in the nest, and feed it as needed. It also seems like you are feeding lots of nutrition-filled items to your birds as well! It is a good idea to have some baby bird food and a syringe on hand that you can mix up if you need to help with feeding, or if the parents aren't caring for it. Most pet stores carry this and will have feeding instructions on the side of the bag. Congrats on the baby bird! It sounds like it and its parents are in good hands.

The easies solution is either Recovery from Royal canin or a/d from hills, both are wet foods designed for cats and dogs but are suitable for baby birds as well, you can draw them into a syringe and feed this bird (many times a day!)

The best situation would be to contact a local animal rescue or control agency that can safely handle and take the bird in for care and treatment, especially if the mother has abandoned it for sure. If the bird seems overheated, moving it to a shaded, cool space or misting it with cool water may help, however if it is in distress and bleeding it is best to be careful when handling it and seek care for it ASAP.

As long as the parents are caring for the babies, I would leave them to do their own thing. I wouldn't move or touch the nest while the babies are still young. Once they're a bit older and moving about, you can then clean the nest, or clean it in between clutches. Having baby bird food and a syringe on hand is always a good idea in case you need to step in, notice they aren't thriving or growing well, or the parents aren't caring for them.