young bird vs baby bird

young bird and baby bird both are nouns.

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

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

Other hyponyms of young bird include fledgeling, fledgling, nestling, eaglet, biddy, chick, cygnet.

young bird (noun)	baby bird (noun)
a bird that is still young	young bird not yet fledged