Aquiline

eagle :: cygnine : ___ : SWAN

Think of a bird known for its hooked beak and sharp vision—this word describes traits associated with it, just as "cygnine" relates to swans.

This clue plays on word relationships using the "::" symbol, which means "is to" or "relates like." "Aquiline" describes things related to eagles, just as "cygnine" relates to swans—so the missing word must be the bird that fits the pattern. Since "eagle" is paired with "aquiline," the blank after "cygnine" is filled by "swan," completing the analogy. It’s a straightforward word-association puzzle where the terms mirror each other.