This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鴜 combines 此 (this/here) and 鳥 (bird), but the exact historical origin and meaning are uncertain. It may be a phono-semantic compound or a variant of another character.
The top 此 suggests 'this' and the bottom 鳥 is a bird. Imagine pointing at a strange bird and saying 'this bird' when you don't know its name, reflecting the uncertain meaning.
For シ, picture a bird sitting on the top of the 此 component: the bird sits, and 'sits' sounds like シ, helping you remember the reading.