Merganser and Smew

Merganser noun - Large crested fish-eating diving duck having a slender hooked bill with serrated edges.

Smew and merganser are semantically related. In some cases you can replace term "Smew" with "Merganser", this nouns are similar.

Smew noun - Smallest merganser and most expert diver; found in northern Eurasia.

Merganser and smew are semantically related. You can use "Merganser" instead a noun "Smew".