young bird vs nestling

young bird and nestling both are nouns.

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

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

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

young bird (noun)	nestling (noun)
a bird that is still young	young bird not yet fledged
	a young person of either sex