large waterfowl with naked head
鶖 refers to a specific kind of large waterfowl with a bare or unfeathered head, such as a cormorant or similar bird.
鶖 is a phono-semantic compound: 鳥 (bird) gives the meaning, and 秋 (autumn) likely contributes the sound. The exact historical development is uncertain.
The top part 秋 (autumn) sits above 鳥 (bird). Imagine a large waterfowl that is most noticeable in autumn, its bare head standing out among other birds.
For シュウ, picture a shoe floating on the water next to the large waterfowl: shoe -> シュウ, and the bird eyes it curiously.