What specific detail must an observer note to differentiate a female Common Merganser from other species?

Answer

The distinct boundary between her rusty-colored head and white neck area.

When observing the female Common Merganser, noting the sharp demarcation where her rusty-colored head color meets the white of her neck area is a crucial feature to help separate her from similar species.

What specific detail must an observer note to differentiate a female Common Merganser from other species?
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