Answer: The capitalized word that is used as an adjective is "Amazon".
Step-by-step explanation: An adjective is a part of speech used to modify a noun or a pronoun. In that way, an adjective offers additional information about a noun/pronoun. In the sentence provided, "Amazon" is working as an adjective because it is describing a noun ("birds"). Furthermore, the type of information that this adjective is presenting about the birds is their origin. As, the word "Amazon" makes reference to a place, the first letter of the word has been capitalized.