answer:It’s a myth. Wildlife rehabilitators (who are often forced to return perfectly healthy animals to perfectly fine situation because humans have decided to interfere) will tell you that handling babies enough to return them to a nest will have no impact whatsoever on the parent. (Perhaps the myth is promulgated in an effort to dissuade people from casually and gratuitously handling wildlife.) I suspect (but have never tested the theory) that a baby that has been handled extensively, been kept in a strange environment, and been fed an artificial diet will smell so different that a mother would reject it if an attempt was made to return it. (Here in my neck of North Carolina, on a daily basis right now, people are actively removing babies from perfectly fine situations because it’s too hot, never mind that these species have evolved to survive in NC. The truly sad thing is that rehabbers are so overwhelmed in this area, they are unable to take in any more animals, so these babies may have been given a death sentance by people thinking that they’re trying to help.)