bird vs bird of passage

bird and bird of passage both are nouns.

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

bird: warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings
bird of passage: any bird that migrates seasonally

Other hyponyms of bird include dickey-bird, dickeybird, dicky-bird, dickybird, cock, hen, nester, night bird, protoavis, Archaeopteryx lithographica, archaeopteryx, archeopteryx, Sinornis, Ibero-mesornis, archaeornis, ratite, ratite bird, flightless bird, carinate, carinate bird, flying bird, passeriform bird, passerine, nonpasserine bird, raptor, raptorial bird, bird of prey, gallinacean, gallinaceous bird, parrot, cuculiform bird, coraciiform bird, apodiform bird, caprimulgiform bird, piciform bird, trogon, aquatic bird, twitterer.

bird (noun)	bird of passage (noun)
warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings	someone who leads a wandering unsettled life
the flesh of a bird or fowl (wild or domestic) used as food	any bird that migrates seasonally
informal terms for a (young) woman
a cry or noise made to express displeasure or contempt
badminton equipment consisting of a ball of cork or rubber with a crown of feathers

bird (verb)	bird of passage (verb)
watch and study birds in their natural habitat