Like babies that take a while before they take their first few steps, young fledglings often cannot fly on leaving the nest.
Popping fledgling birds into a nest or nest box, will only result in these young jumping back out again, these little birds are of a stage of development when all they want to leave home!
Removal of a fledgling from the wild reduces its chances of long-term survival to a small fraction. Fledglings should be left where they are, in the care of their parents. The parent birds are best placed ot teach their young how to master flying and how to find food.
If you find a fledgling on a busy path or road, or other potentially dangerous, exposed location, it makes sense to pick it up and move it a short distance to a safer place.
Birds have a poor sense of smell so handling a young bird does not cause its parents to abandon it but make sure you leave it within hearing distance of where it was found.