sheldrake vs sea duck

sheldrake and sea duck both are nouns.

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

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

sheldrake: large crested fish-eating diving duck having a slender hooked bill with serrated edges
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

sheldrake (noun)	sea duck (noun)
Old World gooselike duck slightly larger than a mallard with variegated mostly black-and-white plumage and a red bill	any of various large diving ducks found along the seacoast: eider, scoter, merganser
large crested fish-eating diving duck having a slender hooked bill with serrated edges