Here's why:
* Common nouns refer to general categories of people, places, things, or ideas. They are not capitalized unless they begin a sentence.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
For example:
* Common noun: animal
* Proper noun: Tiger
"Endangered" describes a state of being, not a specific thing or person. Therefore, it is a common noun.