Question

select the word or phrase that is redundant and can be removed without changing the meaning of the text. new research suggests that plain female birds once had plumage that was as colorful and multihued as that of male birds today.

Explanation:

The word "plain" is redundant. The sentence is about female birds' plumage. The contrast is between their past (colorful) and male birds' present (colorful). "Plain" doesn't add to the core meaning of the comparison of plumage colors. Removing it still conveys that female birds once had colorful plumage like male birds now.

Answer:

plain