young bird vs young

young bird and young both are nouns.

Word	Noun	Adjective	Verb	Adverb
young bird	Yes	No	No	No
young	Yes	Yes	No	No

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

Other hypernyms of young bird include offspring.

young bird (noun)	young (noun)
a bird that is still young	any immature animal
	young people collectively

young bird (adjective)	young (adjective)
	(used of living things especially persons) in an early period of life or development or growth
	(of crops) harvested at an early stage of development; before complete maturity
	not tried or tested by experience
	being in its early stage
	suggestive of youth; vigorous and fresh