cormorant
鶿 refers specifically to the cormorant, a diving bird. The meaning is narrow and concrete, with no clear abstract extensions.
鶿 combines 兹 (likely a phonetic component) with 鳥 (bird) to denote the cormorant. The exact historical development of the top component is uncertain.
The top 兹 looks like a tangled net or fishing line, and the bottom 鳥 is a bird. Picture a cormorant bird tangled in a fisherman's net.
For シ, imagine a cormorant diving into the sea: sea -> シ, and the bird disappears beneath the waves.