Right now the irrigation ditches and ponds have lots and lots of mallard ducklings, sometimes multiple families to a single pond. The way to tell is that there will be too many ducklings for one family in a pond, and they will sort of be grouped up.
I have read that mother ducks will not tolerate strange ducklings around their own, but how do they tell the difference between their own ducklings and unrelated ones?
Most of the birds identify their young by calls. This is a survival trait in birds which allows them to locate their young when they are out of sight. Same applies for ducks as well. As far as I know, the imprinting mechanism (can lead to some hilarious behaviour) that works for the ducklings doesn't have a big effect on the parent to child relationship. In animals, the smell and calls play a major role in parents recognising their offsprings.
