oldwife vs sea duck

oldwife and sea duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
oldwife	Yes	No	No	No
sea duck	Yes	No	No	No

As nouns, sea duck is a hypernym of oldwife; that is, sea duck is a word with a broader meaning than oldwife:

oldwife: a common long-tailed sea duck of the northern parts of the United States
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

oldwife (noun)	sea duck (noun)
tropical Atlantic fish	any of various large diving ducks found along the seacoast: eider, scoter, merganser
a common long-tailed sea duck of the northern parts of the United States