Many years ago when my children were in elementary school, a mother duck and about 13 babies were wandering our area, they got split up as they tried to navigate their ways through all the back yards, and cats outside and peoples dogs. 1 lone duckling got stuck in our back yard and could not find a way out, we could hear the mother call, then the baby make the saddest little cheaping noise.
My children wanted to go and get the little duckling but I told them to wait, surely the mother would come figure this out. This lasted about 4 hours and then the mother and all the other babies disappeared. It broke our hearts listening to this wee duckling "trapped" in our back yard. Finally I could take it no longer and found a little box and the kids and I managed to scoop up the duckling safely.
We called the wildlife vet and they said to bring it to them and they would look after it. When we got there, they told us it is not uncommon for babies to be left behind, the mothers can't always look after all the babies. And if they get split up like that the mother usually won't wait around too long, she has to worry about the ones that are closest to her. The wildlife vet assured us the little one would go to the rehab center and be raised and reintroduced into it's habitat again.