sheldrake vs shelduck

sheldrake and shelduck both are nouns.

Word	Noun	Adjective	Verb	Adverb
sheldrake	Yes	No	No	No
shelduck	Yes	No	No	No

As nouns, shelduck is a hyponym of sheldrake; that is, shelduck is a word with a more specific, narrower meaning than sheldrake:

sheldrake: Old World gooselike duck slightly larger than a mallard with variegated mostly black-and-white plumage and a red bill
shelduck: female sheldrake

sheldrake (noun)	shelduck (noun)
Old World gooselike duck slightly larger than a mallard with variegated mostly black-and-white plumage and a red bill	female sheldrake
large crested fish-eating diving duck having a slender hooked bill with serrated edges