In case you dont get this until later, Turtle is right. You can still return the bird to the area you found it and release it. Find some bushes so the baby feels secure and wont chance getting on the road again.
Even early tomorrow morning would be fine (if its getting too late for you to bike back there now). If it is a fledge, its gonna have nest mates floating around that area too. The parents will find them.
I think most everyone who pays attention to the nature around them has found a baby something and brought it home to try to help. Almost 100% of the time, the parents are nearby and quite aware of their kids condition.
In case you do find an injured animal, or just want to learn more for now (maybe even volunteer) heres a link for MA Wildlife Rehabs:
MASSWILDLIFE Wildlife Rehabilitators
Edit:
Forgot to answer your questions...
Almost 100% of the songbirds take insects in addition to the seeds the finches and sparrows eat. Without knowing what the baby bird is, you could give it a meal of soft bodied insects. You can dig up a couple of worms, catch a dozen mosquitos, smaller spiders (dead), ants that you have killed (they bite) or find other soft bodied bugs.
Dont use crickets or grasshoppers unless you take off their back legs, and only use very small ones. Crickets/grasshoppers must be shorter than the birds beak.
If using worms that are much longer than the birds beak ( 2 X or longer), break them up. There is a chance the bird wont take them from your fingers. Plastic tweezers work, or a toothpick barley holding the food. Sometimes just wiggling a dangling worm chunk will get them to take it. If not you can try brushing the food along side its beak where the top and bottom meet (lips so to speak) gently and sometimes that will get them to take it.
This is a tide it over until morning advice. I would not suggest trying to continue raising the bird and setting it free yourself. I would not (with all I know) attempt to fledge out a bird. I would take it to the pros cuz that is what is in the best interest of the bird (when returning it to its parents is out of the question).
Hope this helps!
