roost vs sit down

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

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

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

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

Other hypernyms of roost include sit.

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

roost (verb)	sit down (verb)
settle down or stay, as if on a roost	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
	take a seat