They do not belong on the ground as a rule. However, there are ground nesting species like quail. These nests are not usually easy to see and a nestling that wanders into view should be left alone as long as there is no obvious injury. Other tree nesting species should be returned to the nest as soon as possible. Birds do not have a strong sense of smell and will NOT reject the baby as a mammal might.
Internal injuries are undetectable and obvious wounds are highly contaminated.
Orphans need continuous nourishment and injuries need proper medical attention. Every minute counts if we are to save a wild life. Keep it quiet. . . Find a quiet place, away from people and pets. Place the animal on a towel, in a box with a secure top.