If you touch a baby bird, the mother will abandon it
Birds have little to no sense of smell, and are dedicated parents who put a great deal of time and energy into feeding and caring for their young. Touching the baby bird will have no effect on the parents' decision to care for the baby wild bird. There is no evidence to suggest that any bird species, including the few who can smell, abandon their young because of the way they smell. Returning a fallen nestling to its nest does no harm at all.