rock wren vs wren

rock wren and wren both are nouns.

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

As nouns, wren is a hypernym of rock wren; that is, wren is a word with a broader meaning than rock wren:

rock wren: wren inhabiting badlands and mesa country of western United States and Mexico
wren: any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects

Other hypernyms of rock wren include jenny wren.

rock wren (noun)	wren (noun)
wren inhabiting badlands and mesa country of western United States and Mexico	any of several small active brown birds of the Northern Hemisphere with short upright tails; they feed on insects
short-tailed bird resembling a wren