Here's why:
* Subject: The subject of a sentence is the person, place, thing, or idea that the sentence is about.
* Verb: The verb is the action or state of being in a sentence ("were" in this case).
* Predicate: The predicate tells something about the subject. In this sentence, the predicate is "were white."
Since the sentence is describing the color of sea feathers, "sea feathers" is the subject.