young vs young bird

young and young bird both are nouns.

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

As nouns, young bird is a hyponym of young; that is, young bird is a word with a more specific, narrower meaning than young:

Other hyponyms of young include hatchling, orphan, young mammal, spat, young fish.

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

young (adjective)	young bird (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