Look closely at the images below.

The two photos on top are common mergansers, and the two photos on the bottom are mallards.

The best way to tell the difference between the common merganser broods and mallard broods is to look at the color of a duckling’s body and head.  Common merganser ducklings (top right) have grey bodies with white patches and a dark rusty-orange head.  Mallard ducklings (bottom right) have brown bodies with yellow patches and a brown and yellow head.

If the ducklings have ANY yellow coloration anywhere, it’s probably a mallard brood.