birdnest vs nest

birdnest and nest both are nouns.

Word	Noun	Adjective	Verb	Adverb
birdnest	Yes	No	Yes	No
nest	Yes	No	Yes	No

As nouns, nest is a hypernym of birdnest; that is, nest is a word with a broader meaning than birdnest:

birdnest (noun)	nest (noun)
nest where birds lay their eggs and hatch their young	a kind of gun emplacement
	furniture pieces made to fit close together
	a gang of people (criminals or spies or terrorists) assembled in one locality
	a cosy or secluded retreat
	a structure in which animals lay eggs or give birth to their young

birdnest (verb)	nest (verb)
gather birdnests	fit together or fit inside
	gather nests
	move or arrange oneself in a comfortable and cozy position
	inhabit a nest, usually after building