young bird vs fledgling

young bird and fledgling both are nouns.

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

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

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

young bird (noun)	fledgling (noun)
a bird that is still young	young bird that has just fledged or become capable of flying
	any new participant in some activity

young bird (adjective)	fledgling (adjective)
	(of a young bird) having acquired its flight feathers
	young and inexperienced