youngster vs nestling

youngster and nestling both are nouns.

Word	Noun	Adjective	Verb	Adverb
youngster	Yes	No	No	No
nestling	Yes	No	No	No

As nouns, youngster and nestling are synonyms defined as:

Other synonyms of youngster include child, fry, kid, minor, nipper, shaver, small fry, tiddler, tike, tyke.

youngster (noun)	nestling (noun)
a young person of either sex	young bird not yet fledged
	a young person of either sex