black-necked stilt vs stiltbird

black-necked stilt and stiltbird both are nouns.

Word	Noun	Adjective	Verb	Adverb
black-necked stilt	Yes	No	No	No
stiltbird	Yes	No	No	No

As nouns, stiltbird is a hypernym of black-necked stilt; that is, stiltbird is a word with a broader meaning than black-necked stilt:

black-necked stilt: stilt of southwestern United States to northern South America having black plumage extending from the head down the back of the neck
stiltbird: long-legged three-toed black-and-white wading bird of inland ponds and marshes or brackish lagoons

Other hypernyms of black-necked stilt include Himantopus stilt, long-legs, longlegs, stilt, stilt plover.

black-necked stilt (noun)	stiltbird (noun)
stilt of southwestern United States to northern South America having black plumage extending from the head down the back of the neck	long-legged three-toed black-and-white wading bird of inland ponds and marshes or brackish lagoons