If you find a bird that looks healthy and energetic but cannot fly, especially in the spring and summer, it is most likely a fledgling.
In the fledgling stage of a bird's life they are at an age where the adults begin to cease the number of feedings, the young then get hungry and hop out of the nest to explore. This increased activity from the young helps build up their flight muscles, which will allow them to fly. The parents will most likely be close-by continuing to feed and protect the animal because this can be a very dangerous stage in their life.
The best thing for you to do is to keep the area as safe as possible by keeping cats and dogs inside, and leaving the area undisturbed. If you feel the area is not safe or a poor foraging area (i.e. a parking lot), or if the bird becomes lethargic, call your local wildlife rehabilitator and discuss how to proceed.