If it fell from a nest under the bridge then it's a cliff swallow. They build nests from mud & twigs. They eat insects. Most rehabers will not take swallows, as they are hard to keep alive. You can place some nesting materials (dried grass or dried leaves, straw) in a deep bucket, like 5 gallon size. Place the chick in it & cover it with a towel. It will need to be fed several (7-10 per feeding) meal worms (heads removed) or 1/2-3/4 inch crickets (again 7-10 per feeding. Freeze the crickets to kill them, then thaw in warm water, drain) 3 times a day. You can get these at places that sell live bait.
If your temps are chilly at night or during the day then you can use a box instead of a bucket & set it on a heating pad. The heating pad should be set on low or medium & be only under half the box. It will need to be kept warm in order to digest food.
The parents will not usually feed them once they are on the ground. They also should be almost as big as the parents in order to fly. Swallow chicks of all species must be able to fly when they leave the nest. It should also weight around 40 - 50 grams. If the keel bone is sharp and or it's stomach skin is wrinkled then it'd dehydrated. Soak the meal worms or crickets in water & feed them wet. Also you need to very carefully pry the beak open & place the food, one at a time, in it's beak. It may eventually grab them. I used long bent tweezers to fed Purple Martin chicks. Swallow chicks should be 26-30 days old before they are ready to leave the nest. If you get it to live, & it's ready to fly, I'd suggest taking it back & tossing it high in the air when adult swallows are there. They may help it stay airborne & it's parents may even reconnect with it.
If you feed the meal worms & crickets wet that's enough water for it.
Best wishes.
