Adult barn swallows spend most of their time outside the nest, hunting. They probably have not abandoned the nest. Likely they will come back (bearing food).

(I like barn swallows. They don't make much mess, they are beautiful in flight, and they eat bugs.)

Yes, the parents will feed it.

A family of barn swallows has a nest outside my window. Today, I found one of the young (feathered) birds on the ground. Assuming he was injured, I took him inside to save him. However, after reading some info online, I see he may have just been learning to fly and perhaps wasn't injured. I took him back outside and placed him in his nest. It's been a while now though, and I still haven't seen the mom come back to feed him nor have I seen any of his brothers/sisters. He's still just sitting alone in his nest. Will his mom come back to care for him or should I take him to the humane society?