air sac vs bird

air sac and bird both are nouns.

air sac (noun)	bird (noun)
a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways	warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings
any of the thin-walled extensions of the tracheae of insects	the flesh of a bird or fowl (wild or domestic) used as food
any of the membranous air-filled extensions of the lungs of birds	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

air sac (verb)	bird (verb)
	watch and study birds in their natural habitat