migration vs gold rush

migration and gold rush both are nouns.

Word	Noun	Adjective	Verb	Adverb
migration	Yes	No	No	No
gold rush	Yes	No	No	No

As nouns, gold rush is a hyponym of migration; that is, gold rush is a word with a more specific, narrower meaning than migration:

Other hyponyms of migration include emigration, expatriation, out-migration, immigration, in-migration.

migration (noun)	gold rush (noun)
the movement of persons from one country or locality to another	a large migration of people to a newly discovered gold field
the periodic passage of groups of animals (especially birds or fishes) from one region to another for feeding or breeding	a sudden happening that brings good fortune (as a sudden opportunity to make money)
(chemistry) the nonrandom movement of an atom or radical from one place to another within a molecule
a group of people migrating together (especially in some given time period)