Well, in many places they are protected so that the species can thrive without interferance from people. In FL, where I used to live, bothering mallards or their nests was a felony. Unless you are a wildlife rehaber, you will not be able to teach a duckling, wild or not, the skills that it needs to survive. Migration is a learned behavior that mother ducklings teach their young. Predator evasion, foraging and appropriate food stuffs are also taught.
Because of the behaviors of many, some species are more affected by people than others. People release domestic ducks into the wild and diseases not present in wild populations can be introduced. Domestic ducks breed with mallards which pollutes the gene pool. Domestic mallards and wild mallards are different. Wild mallards are stream lined and smaller than domestic mallards. Domestic mallards have been bred to be larger and produce more young. More young in the wild population increases pond density, decreases food supply and can have a devastating effect on the entire population.
Regardless, the duckling would have slim chances of survival in the wild being raised closely by a person.
