Here's why:
* Collective nouns represent a group of things. "Cages" can refer to multiple individual cages, making it a collection.
* They can be singular or plural. You can say "the cages are empty" (plural) or "the cages are in the back room" (singular, referring to the group as a whole).
However, it's important to note that "cages" is not *always* a collective noun. If you are talking about individual cages, it's simply a plural noun.
For example:
* Collective noun: "The cages were all locked." (Refers to a group of cages)
* Plural noun: "There were five cages in the animal shelter." (Refers to individual cages)