sit down vs roost

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

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

As verbs, roost is a hyponym of sit down; that is, roost 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
roost: sit, as on a branch

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

sit down (noun)	roost (noun)
	a perch on which domestic fowl rest or sleep
	a shelter with perches for fowl or other birds

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