Why did our feral baby bird died?

It's impossible to say for sure what caused the birds demise.. it may have been injured in the original fall from it's nest, it may not have been kept warm egougn, or it may have died of starvation. Baby birds are like any babies. They need warmth, and they eat often. I've raised many outdoor birds including sparrows, mockingbirds, finches, a hummingbird, and starlings. In fact one starling still lives with me. He is now 14 years old. All of the birds came to me in much the same way this little guy came to you.. Here's a list of supplies to keep on hand, and what to do if you think you may try this again.. get at least 2 syringes (the kind you get a shot with, but no needle of course), at least one jar of strained vegetable beef baby food, liquid bird vitamin drops, and a small bird cage with one wood perch close to the bottom of the cage. Nurses are a good source for the syringes, or some pet/feed stores may carry them, and walmart carries caged bird vitamin drops for about $3. OK, so put the little fella on soft padding, like fleece, (don't use towels, or 'nappy' fabric, as his toenails can get caught in the nap) in a small box with loose fitting lid. You can poke a few tiny holes in the top if you want. Mix about 2 tablespoons of babyfood and a 2 drops of the vitamins into a tiny container. Use syringe to suck up the mixture. I usually prepare 2 -3 syringes at once, then put the rest in the refrigerator. Keep the syringes of food out at room temp. If the baby is very young.. it will have very few, if any feathers, and will need to be fed every 2-3 hours day and night for the first few days. Do not try to give him water. There is sufficient water in the babyfood. Now, to feed him.. OK, so pry his beak open with fingertips of one hand, and with the other just stick the syringe in a bit and gently push in a bit of the food.. he'll swallow it. Shouldn't take more than a few tries before he's begging for more. Keep him inside, in a cozy spot free of drafts, extra cover at night if you're in a cold climate. Within days he'll be eating more at once, and sleeping through the night.. Within 1 to 2 weeks, introduce him to the little bird cage, bird feed appropriate for his species, while supplementing the babyfood, until he's finally eating on his own, and introduce him to a bird water dish. The rest is up to you.. are you going to keep him as a pet? if not, next you begin the outdoor acclimation process, and that's another whole story.. By now, you should have a whole new respect for parent birds. They work their little tushes off, and unlike us humans, they give 100% 24/7 parenting until their little ones are ready to spread their wings and fly away.. that is unless the little brats fall out of the nest, or try to take off before they're ready. Good Luck!