Caring for an Orphan Duck?

I found a baby duck this morning in a busy street by our church - no where near any place ducks would live. The baby sits easily in the palm of my hand. It is roughly half the size of a dollar bill. After an hour of looking, we went to a RV park, a good distance from our church, that had a pond with one duck. The manager said the lone duck would kill the baby and they had no other ducks. She then stated that about 2 weeks ago, a female mallard arrived there and the lone duck was mean to it, so the lady took the female home. I am wondering if it would be possible that the mallard had a nest and in this daily 100 degree weather we've had, this baby hatched from that nest? Ideas?