bird vs nester

bird and nester both are nouns.

As nouns, nester is a hyponym of bird; that is, nester 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
nester: a bird that has built (or is building) a nest

Other hyponyms of bird include dickey-bird, dickeybird, dicky-bird, dickybird, cock, hen, night bird, bird of passage, 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)	nester (noun)
warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings	someone who settles lawfully on government land with the intent to acquire title to it
the flesh of a bird or fowl (wild or domestic) used as food	a bird that has built (or is building) a nest
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)	nester (verb)
watch and study birds in their natural habitat