kind of water bird
鸂 refers to a specific kind of water bird, likely the mandarin duck or a similar species. Its meaning is narrow and concrete, centered on that particular bird.
鸂 combines 溪 (mountain stream) and 鳥 (bird), suggesting a bird associated with streams. The exact historical development is uncertain, but the component choice likely reflects the bird's habitat.
A bird (鳥) found by a mountain stream (溪): 鸂 is that water bird, like a mandarin duck living along clear streams.
For ケイ, picture a kayak gliding down the stream where the water bird lives: kayak -> ケイ, and the bird watches from the bank.