rare bird vs worker

rare bird and worker both are nouns.

Word	Noun	Adjective	Verb	Adverb
rare bird	Yes	No	No	No
worker	Yes	No	No	No

As nouns, worker is a hypernym of rare bird; that is, worker is a word with a broader meaning than rare bird:

Other hypernyms of rare bird include actor, doer.

rare bird (noun)	worker (noun)
a rare or unique person	sterile member of a colony of social insects that forages for food and cares for the larvae
	a person who works at a specific occupation
	a member of the working class (not necessarily employed)
	a person who acts and gets things done