foster-child vs nestling

foster-child and nestling both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster-child	Yes	No	No	No
nestling	Yes	No	No	No

As nouns, nestling is a hypernym of foster-child; that is, nestling is a word with a broader meaning than foster-child:

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

foster-child (noun)	nestling (noun)
a child who is raised by foster parents	young bird not yet fledged
	a young person of either sex