sit down vs perch

sit down is not a noun while perch is a noun.

Word	Noun	Adjective	Verb	Adverb
sit down	No	No	Yes	No
perch	Yes	No	Yes	No

As verbs, perch is a hyponym of sit down; that is, perch is a word with a more specific, narrower meaning than sit down:

sit down: be in a position in which one's upper body is largely upright and supported by one's backside
perch: sit, as on a branch

Other hyponyms of sit down include lounge, sprawl, rest, roost, crouch, hunker, hunker down, scrunch, scrunch up, squat.

sit down (noun)	perch (noun)
	spiny-finned freshwater food and game fishes
	any of numerous spiny-finned fishes of various families of the order Perciformes
	support consisting of a branch or rod that serves as a resting place (especially for a bird)
	any of numerous fishes of America and Europe
	an elevated place serving as a seat
	a linear measure of 16.5 feet
	a square rod of land

sit down (verb)	perch (verb)
show to a seat; assign a seat for	sit, as on a branch
be in a position in which one's upper body is largely upright and supported by one's backside	cause to perch or sit
take a seat	to come to rest, settle