brown thrush vs mocking thrush

brown thrush and mocking thrush both are nouns.

Word	Noun	Adjective	Verb	Adverb
brown thrush	Yes	No	No	No
mocking thrush	Yes	No	No	No

As nouns, mocking thrush is a hypernym of brown thrush; that is, mocking thrush is a word with a broader meaning than brown thrush:

brown thrush: common large songbird of eastern United States having reddish-brown plumage
mocking thrush: thrush-like American songbird able to mimic other birdsongs

Other hypernyms of brown thrush include thrasher.

brown thrush (noun)	mocking thrush (noun)
common large songbird of eastern United States having reddish-brown plumage	thrush-like American songbird able to mimic other birdsongs