wren vs cactus wren

wren and cactus wren both are nouns.

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

As nouns, cactus wren is a hyponym of wren; that is, cactus 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
cactus wren: large harsh-voiced American wren of arid regions of the United States southwest and Mexico

wren (noun)	cactus wren (noun)
any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects	large harsh-voiced American wren of arid regions of the United States southwest and Mexico