down feather vs plumule

down feather and plumule both are nouns.

Word	Noun	Adjective	Verb	Adverb
down feather	Yes	No	No	No
plumule	Yes	No	No	No

As nouns, plumule is a hyponym of down feather; that is, plumule is a word with a more specific, narrower meaning than down feather:

Other hyponyms of down feather include duck down, goose down, swan's down.

down feather (noun)	plumule (noun)
soft fine feathers	down feather of young birds; persists in some adult birds