white-headed stilt vs longlegs

white-headed stilt and longlegs both are nouns.

Word	Noun	Adjective	Verb	Adverb
white-headed stilt	Yes	No	No	No
longlegs	Yes	No	No	No

As nouns, longlegs is a hypernym of white-headed stilt; that is, longlegs is a word with a broader meaning than white-headed stilt:

white-headed stilt: stilt of the southwest Pacific including Australia and New Zealand having mostly white plumage but with black wings and nape of neck
longlegs: long-legged three-toed black-and-white wading bird of inland ponds and marshes or brackish lagoons

Other hypernyms of white-headed stilt include Himantopus stilt, long-legs, stilt, stilt plover, stiltbird.

white-headed stilt (noun)	longlegs (noun)
stilt of the southwest Pacific including Australia and New Zealand having mostly white plumage but with black wings and nape of neck	long-legged three-toed black-and-white wading bird of inland ponds and marshes or brackish lagoons