Merganser challenge

Common mergansers often will show up among groups of red-breasted mergansers, especially when they appear in salt water. In the photo below, note the chestnut head with a well-defined white chin patch and well-defined contrast between the head and neck color on the common merganser, which is the top bird. Despite its name, the common merganser is the most uncommon merganser to visit the Carolinas. This bird has been present at Pawley's Island, SC for a few weeks.

Common and Red-breasted Mergansers by Pam Ford

The red-breasted female on the bottom lacks the definition of the chin patch and contrast between the head and upper breast. The plumage colors gradually mix and change from dark to light around the head and neck area.  The red-breasted also has a thinner bill and a longer, shaggier crest.