bunk down vs bed

bunk down is not a noun while bed is a noun.

Word	Noun	Adjective	Verb	Adverb
bunk down	No	No	Yes	No
bed	Yes	No	Yes	No

As verbs, bed is a hypernym of bunk down; that is, bed is a word with a broader meaning than bunk down:

Other hypernyms of bunk down include crawl in, go to bed, go to sleep, hit the hay, hit the sack, kip down, retire, sack out, turn in.

bunk down (noun)	bed (noun)
	a piece of furniture that provides a place to sleep
	a plot of ground in which plants are growing
	a foundation of earth or rock supporting a road or railroad track
	the flat surface of a printing press on which the type form is laid in the last stage of producing a newspaper or magazine or book etc.
	single thickness of usually some homogeneous substance
	a stratum of ore or coal thick enough to be mined with profit
	a depression forming the ground under a body of water
	(geology) a stratum of rock (especially sedimentary rock)

bunk down (verb)	bed (verb)
go to bed	prepare for sleep
	have sexual intercourse with
	put to bed
	place (plants) in a prepared bed of soil
	furnish with a bed