merganser
Refers to any duck of the genus Mergus, a diving duck with a slender serrated bill. In everyday contexts, the katakana form アイサ is most common.
The merganser is a waterbird skilled at catching fish.
In winter, mergansers come to this lake.
smew
Katakana spelling is common for this bird name; usually written in kana.
Kanji spelling is rare and may not be widely recognized; the reading あいさ is standard.
The Japanese name アイサ (aisa) is of uncertain origin. It may be derived from the bird's call or from an older Japanese word, but the exact derivation is unclear.