I can't believe that before 2 years ago, I'd never even seen a baby bird before, even a bird nesting in a tree, or babies in that nest, and now I'm FEEDING baby birds of all size and species --all the time. Some eat about every 20 minutes!
We feed them Science Diet cat food, which come in perfect sized pellets, soaked in water to make them the right consistency. It's packed with nutrients. The tiniest babies can swallow them whole, though I do break them apart and make sure to swish them between my fingers so it's more like what their mother might bring. Depending on the species, we may also feed them bits of berries and/or crickets (some with legs, some without), and meal worms. All of these are extended via a kind of tweezer that replicates a mother's beak, which we sterilized between uses per bird.
Here's a peek at tiny new born chick and two different sized mourning doves that someone brought in. I was transferring them from Triage to a nice, warm "nest" in tanks holding others of their own kind. Once settled, I'd start their feedings.
There are many mourning doves in a long row of tanks and those need to be tube fed with a liquid mixture that resembles more how their own mothers feed them. I'd describe it better, but you might be checking this over lunch or with your first cup of coffee and, well, it's better left in these terms.
When the birds are ready, they get transferred to the mews we have just outside, where we have propped branches and put in a bird bath. There they have lots of company and can get used to short flight and fresh air and community. After that, they go to a rehabbers home, where there is the acreage to build a flight cage. They can catch insects and hunt on the ground for food, learning from the other birds. It's the last step before the birds are released and fly free to enjoy the lives that were saved by these Centers, and go on to have babies of their own.