wren vs winter wren

wren and winter wren both are nouns.

Word	Noun	Adjective	Verb	Adverb
wren	Yes	No	No	No
winter wren	Yes	No	No	No

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

wren: any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects
winter wren: small wren of coniferous forests of Northern Hemisphere

wren (noun)	winter wren (noun)
any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects	small wren of coniferous forests of Northern Hemisphere