Mergus albellus vs sheldrake

Mergus albellus and sheldrake both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mergus albellus	Yes	No	No	No
sheldrake	Yes	No	No	No

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

Mergus albellus: smallest merganser and most expert diver; found in northern Eurasia
sheldrake: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of Mergus albellus include fish duck, merganser, sawbill.

Mergus albellus (noun)	sheldrake (noun)
smallest merganser and most expert diver; found in northern Eurasia	Old World gooselike duck slightly larger than a mallard with variegated mostly black-and-white plumage and a red bill
	large crested fish-eating diving duck having a slender hooked bill with serrated edges