flock vs bunch up

flock is a noun but bunch up is not a noun.

Word	Noun	Adjective	Verb	Adverb
flock	Yes	No	Yes	No
bunch up	No	No	Yes	No

As verbs, bunch up is a hyponym of flock; that is, bunch up is a word with a more specific, narrower meaning than flock:

Other hyponyms of flock include huddle, huddle together, bunch, bunch together.

flock (noun)	bunch up (noun)
a group of birds
a group of sheep or goats
a church congregation guided by a pastor
an orderly crowd
(often followed by `of') a large number or amount or extent

flock (verb)	bunch up (verb)
come together as in a cluster or flock	form into a bunch
move as a crowd or in a group